i don't know if you have wraps but they provide the support you need. if you don't have some you can get them at most sporting goods stores i think. you can even get them at a super wal-mart. they're about $4. don't worry if you don't know how to wrap them because i think there's a description on this site and i think there's directions in the package. if you keep hitting the bag you'll learn to stabalize your wrists and i guess they get stronger. just in the begining don't go nuts and hurt them. i did, not seriously though, i think everyone does cause they're excited.
also, about conditioning, you might want to skip some rope. you can get a speed rope (which just means it's made of plastic so it goes faster) at wal-mart. i think they're $6. so together that makes $10. not bad. just don't get frustrated. it gets to be fun and kinda relaxing the better you get at it. there's a good description of rope-skipping workouts at prcticalfightingconcepts.com.
i wouldn't worry about suppliments, but i am not very knowlegable on suppliments so that's just me. i used to take protein and amino acid chains and it really helped with my recovery time but i don't think it was worth the money. i didn't buy any again so that speaks for itself.
i think the best place to start if you have the money is with someone who knows what they're doing and can show you, like a trainer. but if you don't have the money there's nothing wrong with teaching yourself. go to the library or a used book store and find a book on boxing techs. then practice them and ask these guys questions as they pop up because they're always happy to answer mine. but then go back and read them again because there always seems to be details you'll miss. at least that's how it is for me; this is how i taught myself to box. well mostly, i went to a kickboxing jiu jitsu gym every once in a while and learned something everytime i went. but mostly from books and sites like this one. but again if you have the money i think the best place to start would be with a trainer. some of the guys on this site were talking about a site called rossboxing.com i think. i liked some of the workouts he had so that may be a good source for you.
also about your wrists, try tightening your fists. it may help them to keep from buckling.
i hope that helps and i hope i didn't tell you anything you knew already. if i did sorry about that.