I would like to buy a softball bat to play, but dont know how to choose them, the right size and/or weight.

Are you already playing on a team? If so, start off by using one of your teammates bats until you find something that you like. No point in spending a couple hundy on a bat if you don't know if you'll like it, or not. Some bats are "classified" as the same weight (ie: 28 oz) but the weight is proportioned differently on the bat. One bat may feel like the weight is on the end of the bat, where another may feel like the weight is closer to the handle. It's ll in what you prefer and what feels good for YOU.

Try swinging it it might help An dI think it a close to the waist.

30, 31,32 inchs is a good size

it all depends and your arm strength. Don't use a larger bat than you can swing compfortable, however, u want to use a large of a bat as you get without sacrificing a significant amount of bat speed. Hope this helped

for the length put the bat by your side, stand straight, your wrist should go just below know knobby thing on the end at the end of the handle.(sorry don't know what the proper term is)
for the weight-try swinging it its usually all about the balance I like my bat to be almost equal weight between the barrel and the handle. It will probably be closer to the barrel.