I understand that newborn's immune system is still weak and crowds/strangers are to be avoided. Knowing this, how soon can I take my newborn outside, for example to take ~1 hours walk in a non-crowded park? Any particular things to pay attention to when doing this?
This pamphlet, put out by the South Carolina Department of Health, suggests it is safe at one month. You should avoid large crowds, but the fresh air and stimulation is good for baby. You should probably not allow people to touch your baby's hands (since baby may put them in his mouth). Your greater concern in going outside is the weather - keeping baby warm/cool enough and avoiding too much direct exposure to sun.
Personally, in a non-crowded situation, I would not hesitate to take the baby for a walk as soon as I felt up to it - usually within days of birth.
The guideline I've been given by our pediatricians was to avoid anybody with an easily transmittable disease for the first six weeks. The main reason to stay away from crowds and/or strangers is that you don't really know who in the area is ill or contagious. You don't want somebody to sneeze in your kid's face at any age, really, but a newborn is more vulnerable than a toddler.
To more directly answer your real question: We took our daughter for short walks through our favorite nearby woods from when she was only a week old. I don't think she necessarily got a whole lot from the experience at that age, but it was great for me to get some fresh air and exercise. As long as the baby is comfortable and adequately protected from temperature/sunshine, you can take her wherever you want to go.
I have never heard that strangers are to be avoided with newborns, and I certainly wouldn't recommend staying inside to avoid them. You need to get out for your own sanity, so get out and go for walks as soon as you can. Exposure to new environments and people is good for babies development, keeping them cooped up indoors and away from social contact is not the recipe for a happy child. As for any immune system concerns the only way for them to get stronger is to build up resistance, they may as well do it while having fun. If you know someone is sick keep them away by all means, that's just good sense, but going overboard is not helpful.
Somebody asked the same question in a new parents support group we were in after the birth of our son. The answer given was, "You already took your baby outside when you went home from the hospital. Find something else to worry about."
Being handled by random people is a different story, but merely being outside the home (assuming adequate clothing and sun protection) doesn't appear to have any good basis for avoiding it.
We took our newborn daughter out to a crowded shopping center, on a busy Saturday, at 3 days old. And being our first child, we were being overly careful with her as only a new parent is!
Shock/Horror nothing happened. She grew up fine. You should be careful with newborns, they're immune system may not quite be at full power yet, but equally they don't need to be cocooned either.
Here in Asia, people are pretty conservative. They don't take babies out until they are 3 months old. We followed that and my child seemed to better than others (perhaps just lucky).
I think the issue with the under-developed immune system is good and it is very hard to tell who has an asymptomatic infection disease. So, for my child, I prefer to fall on the conservative side. That said, we did have friends and family over during those first three months.
For me it really depends on the weather in your region of the world. We went for longer walks progressively, and when the weather was better (no rain, snow, ice or excessive heat).
We live in the North of France where it is freezing during winter so we didn't really get outside more than a few minutes with our newborn until he was 2 or 3 months old.
On the other hand, our daughter was born during summer when the weather is really comfortabl. After a few weeks for the mother to recover we went out progressively for 20 min to about 1 hour when she was 1 or 2 months old.
Oh, and it seems to be recommended to use one extra layer of clothes for the child since he is not walking himself.
Protecting from direct sun rays is also obvious.