The most important thing to do for a good experience is to get on to a team that fits what you are looking for. There are teams built to win at all costs-mandatory practices, best players get to play, if you lose a lot you sit, ect.
Then there are teams that are completely social where there are no practices, everybody plays and you go out and have drink afterwards. There are all kinds of variations in between. Find a good group of guys that fit.
There are some very competitive people out there but I wouldn't let that deter you. That actual number of outright jerks is pretty low. But there is no doubt that you will encounter some hooking and bad gamesmanship.
Don't worry about your self rate unless you were a former college or high school player then you have to rate at a certain level depending on what you previously did. You will get bumped or you can move yourself up the following season.