Position is extremely important in Texas Hold'em. Your absolute position is where you currently are in relation to the button and it changes every turn. Best position is late position, the button: you can act last in a betting round and you will have the most information on the actions of other players. The worst position is on the left of the button or 'early position' because you'll have to act without any information at all. The number of hands that can be played safely from an early position is therefore quite limited. Players in early position lack the necessary information for calculating their pot odds and their bets can be raised by players in later positions.
Your 'relative position' (=position in relation to other players) is important as well. If you have a very aggressive player at the table that raises a lot, try to sit at his left. By doing so, most times you will be able to see his raises before it is your turn to act, allowing you to drop mediocre hands.
When do you Hold'em, when do you fold'em? The first thing to learn is judging the winning potential of your pocket cards correctly. There are exactly 1326 equally probable starting hand combinations in Texas Hold'em, but as all suits are ranked equal there are only 169 possible unique starting hands.
Tells will help you assess the hand of your opponent. Some possible tells include leaning forward or back, placing chips with more or less force, talking while otherwise silent, fidgeting, doing chip tricks, changes in breathing, facial expressions, staring, ... .
Be careful though, a tell can be faked hoping to induce the opponents to make false assumptions in response to the tell.