With respect to what hands you play and why, I suggest that you get a feel for how strong a hand is versus another hand. For example you need to know that a medium pair versus two high cards is about a coinf flip etc. Mess around with poker calculators and put in several scenarios. Such as 3 people holding ABC cards with a flop of xyz and you will be able to see how strong or weak a hand is. In general, you need strong to very strong hands in early position to call the big blind. In middle positions, the requirements are relaxed a bit. In late position, you may limp in with mediocre hands PROVIDED THAT YOU ARE GETTING THE PROPER POT ODDS
TO DO SO. If you have a hand like 7/8 suited and you are on the button, I would call if 5 people limped in because you will be getting 6 to won on your money. Understand that most of the times, you will miss the flop so you get out, but when you do connect and win, the implied odds that you get will justify that call. On the other hand against only two other people, I would fold because chances are that the other two have high cards especially if they are not in the blinds. Your calling requirements will also dictate your fluctuation. If you call with many hands, you will win more pots but you will also experience more swings in your bankroll. Loose aggressive = Huge Swings Tight = Less fluctuation. It's the classic risk reward model.