Bullying is child abuse. If someone hits you, it doesn't make any difference how old the fist is, it still hurts. The same goes for psychological abuse. Unfortunately some adults still don't seem to realise this.
Bullying is not something that a child should have to deal with on their own, so "bully-proofing" and teaching them "how to deal with bullies" isn't really the point. If you are a victim of bullying then there is no good way of dealing with the perpetrators, and telling the child that there is some recipe for solving the problem is basically just victim-blaming. The only real solution is for adults to step in and deal with the bullies.
The only real thing you can do is encourage your child to tell you if things are going badly, and to teach them that bullying is wrong. Calling people nasty names, hitting, taking their things. Teach your son that if anything like this happens then they should first tell a teacher, and then tell you at the end of the day. However be careful that you don't scare him with all the things that nasty children might do.
Unfortunately if the child won't say anything its really hard to tell the difference between being bullied, being a bully, and normal everyday stuff. Its not nice to think about, but you need to be equally on alert for signs that your son is being a bully as well as signs that he is being bullied. This US government web page has some general advice on what to watch out for.
The first step is to check with the school. You don't say which country you are in, but in most places schools are required to have an anti-bullying policy. Ask for it (the school website is a good place to start) and see what it says. If they don't have one on their website then ask specifically what their policy is. If your school has a PTA or similar organisation then ask them too.
Once your child starts school talk to them about how their day went and what they did. Don't ask "did anyone hit you", ask open questions about what happened. Ask about what happened during break/recess. Especially during the early days your son should be happy to share everything. If he suddenly shuts down about e.g. lunch time and says "nothing" then there may be a problem, but don't assume it is bullying; it could just as easily be that he was told off by a teacher for something.
Find out if your son is making friends, and encourage stuff like play dates. Children with friends are rarely victims of bullying; its the loners who get picked on. Also if you invite his friends over you can look for any negative patterns in their interactions; some children put up with bullying by "friends" as the price of admission into the group (and again, look out for signs that your son is a perpetrator as well as a victim).
Chat to other parents at the school gate. Ask how their children are doing. Swap stories. If bullying is a general problem at the school then this is a good place to find out.
In theory your son's school should be ahead of you on all this; they can observe his behaviour and interactions, and they should be following their own anti-bullying policy. Unfortunately sometimes the policy gets written, stuck on a shelf, and forgotten. If you think this is happening to your son then ask for a meeting with the teacher, take a copy of the anti-bullying policy with you, and ask some pointed questions.