I have a sister, and she doesn’t believe in god. But she is financially independent and is much older than me. I feel filthy every single time I pray and really need a way to tell my parents I’m not Christian. But I’m afraid that they’ll force me to pray as they probably would think that it’s just a phase. I also really want to go to a music concert and the songs the band sings have loads of swear words in them so I would never be allowed to go if they thought I was Christian.
I feel filthy every single time I pray
Prayer shouldn't make you feel this way even as an atheist. A good way to think of prayer, even if there's no divine to commune with, is simply as a moment of quiet contemplation, a moment of rest. You don't have to speak the words if they don't have meaning for you, but especially now, that moment of silence is useful in difficult times.
If your parents are into the big ritual of spoken prayers, simply tell them that you're choosing to have a moment of silence (true), and mean no disrespect (also true).
Then honestly, half the time, I just thought about people I hadn't seen in a while, and the other half, I thought about the videogame I'd been playing before church.
I also really want to go to a music concert and the songs the band sings have loads of swear words in them so I would never be allowed to go if they thought I was Christian
I don't think coming out as an atheist would affect this decision for them, if anything they'd be more concerned about it taking you down a path they're uncomfortable with. The best way to persuade them to allow you would be to ask what you could do to earn it. If you can find a fellow classmate (preferably one with good grades whom your parents like) who is going, don't talk about the band, talk about going with them.
In this case I would also talk to your sister. She will understand your parents better than we can, have her own experiences, and should hopefully be able to give you more specific guidance.
[Edit: I noticed that my answer was pretty abstract, and that I was writing for someone who will need a few examples. So throuout these post, you will find a few "Edit"s that I added after writing this.]
First step: Priorities
Before you decide on how to tell them, I would advice to think about your priorities. How you and your parents handle this conflict will influence the rest of your life. The concert will likely not influence the quality of your life 5, 10 or 40 years from now.
So, what is your goal?
Your parents not harrassing you?
Your parents actually accepting your conviction?
Just not having to lie and pretend whenever you are at a service or praying or in a religious discussion?
Being allowed certain things you are not allowed to do under some religious reason?
Being allowed to attend this concert in x days, no matter what?
[Edit: Example: If you just want your parents to accept your stance, you will want to start slow, telling them that you have doubts. Or asking them what makes them believe. If you just want to stop pretending but don't worry about their feelings and their acceptance, telling them can be done in two (polite) matter-of-fact sentences.]
Second step: Consequences
Consider the fallout. Talk to your sister about it, but understand that she was older when she did it. I do not know how "radical" your parents are. Worst-case scenario you might end up at some religious re-education-camp and/or school. Remember: From their point of view, god will have you tortured for eternety if you have doubts his existance. So they might willingly harm you just to "save" you from that. And since you are twelve years old, they have nearly absolute power over you. There is a whole spectrum of other consequences that could make your life worse (or better). But a lot of the consequences will depend on your approach. So consider them.
[Edit: If it might make your next few years horrible, or you have to fear that they will not support college or cut you off in some other way, consider to wait a few years. In that case your "lies" would be more their fault than yours. If you expect them to send you to some pastor or priest right away, you might research the typical arguments and tricks someone with that amount of training might use in a "discussion". If you expect some other specific fallout, think about how different approaches mit influence that.]
Third step: How To Do It
Generally, I advice to approach this in a non-offensive way that includes a lot of communication short and long-term. It is easy to be understood as: "Your believes are stupid - and if you do not realize that, you must be stupid, too!" Tell them that you are sorry, but you have not found any sensible reason why you should believe. Talk about that topic in a constructive way. Be prepared that sooner or later your parents or someone else will bring some argument you might doubt, but cannot yet counter. If you have approached those conversations in a constructive way (and not like a you-vs-me "discussion"), you will be able to say "Good question. I will think (and research) about this."
[Edit: "Approaching a conversation non-offensively" is not about offensive (swear-)words. You can google it, but some ways are:
Use "I" messages. "I feel like I am lying when we are praying." is better than "You always force us to pray". That is usefull because it does not seem to blame the other person.
Ask honest questions and listen. Not to find the best counter, but to understand. And, slowly, try to make your parents understand as well. The US has a culture where a public "debate" is about "scoring points" and hurting the "opponent". Most other "small-talk" is expected to be non-political, non religious and so on. So you (and your parents) might not be used to this kind talk. But try it.
Oddly enough, you might need to be the adult here. They might have strong emotions - make sure that they do not trigger yours. Do not be cold, but stay friendly and understanding even when they get loud, threatening, passive-agressive and so on.
Remember that not everything needs to be said in the first conversation. First priority should be to enable further talks, only second is to communicate all that you feel the need to tell them.]
You might want to seperate these issues. Even non-christian parents might not allow you to go there. There is a saying "choose your battles". This might be a time to do that, and focus on one issue at a time.
You might decide that you want to wait with the religion-topic and focus on the concert, though. If you do so, you might point out that the bible is not really about non-swearing. It advices not to miss-use Gods name, and that some passages have a lot of negativity when it comes to sex. But swearing in general is not really a focus of the bible's ethics.
But best double-check that: The bible is a complex book with all kinds of stuff in there... it might be a quote about swearing somewhere that I missed.
I know how you feel. People told me "it cannot hurt", but I usually answered: "Yes. But it feels like lying to me, and I do not like to lie." When parents/family pray at dinner or in some other context, I politely bow my head and stay quiet. Out of courtesy and to avoid conflict. I have my hands in my lap but would never fold them.
If you choose not to tell your parents yet, there are different ways to approach it. For silent prayer, you might want to look into meditation. Not the spiritual kind, but the kind people use to be more focused on their job. Ordering your thoughts on a regular basis is a pretty powerful tool after all. "Real" prayer actually offers just that benefit: A person who "tells god" their most important goals and problems daily will likely keep them in mind while doing decisions during the day.
[Edit: Google for the "Eisenhauer Matrix". You could make it a habit to mentally sort your tasks and/or goals for the day in that manner whenever it is time for silent prayer.]