[papawawa]

As a relatively new player to the game, i have experienced some of the tendencies you've mentioned. Here's what I think:
-You losing to one particular hand or another is not the problem; its that you are oblivious to the POSSIBILITY that a better hand has been made. My advice would be to pay more attention to both the board and the betting patterns of your opponents. You have to charge people to "make straights on garbage hands" as well as fold decent hands when you realize you've been sucked out on. If you start applying this you will see an immediate improvement in your game.
-This is a such a vague question, you should really just read a book about it. The abridged version: It depends on (1)what you cards are, (2)your postion relative to the raiser, (3)and what you think of that raiser. The better your cards are, the more likely you are to call the raise. If you are in a better position and the raiser is good, be more inclined to fold; however in this same situation if he plays badly, be more inclined to call or even reraise. Finally, if the raiser is extremely loose, definetly at least call.
-Yes, pot odds are extrememly important. Yes, it is possible to estimate pot odds, although there really isn't a need when playing online - the pot size automated. And yes, you should be relatively precise in your pot odds calculations, again not that difficult on the PC.
My final bit of advice is to go out and get a book on the subject. A good one for beginners is Hold'em Poker by Sklansky. Good luck and wecome to the forum.