I know when to fold in Texas Hold Em. But, I don’t know when to call and when to bet. I’ve read a LOT of conflicting information about when to do these things.
The easiest thing to say about calling is “call when the pot is giving you better odds than you think your chance of winning is.”
For instance, if you are 4-1 against hitting your flush, and the pot is giving you 5-1 AND you know that your flush will win, it’s an easy call. If you don’t even understand that sentence, you have some reading to do.
Sometimes it’s that easy. More often than not, you need to estimate your chances of winning based on your estimate of your opponent’s hand strength.
Also, when calculating what the pot is paying you, you also need to take into account the amount of money you may win from your opponent in future betting rounds. For instance, in the example above, you don’t need sufficient “immediate odds” from the pot if you know that your opponent will pay you more money if you hit your flush (“implied odds”).
There’s a lot more to take into account, but that’s the basic idea.
A very useful thing to keep in mind about betting: never bet if your opponent will only call with a hand that beats you. (for instance, if you have a pair of deuce’s on the final betting round).
Beyond that, you’re asking a question (when to bet?) that has taken up thousands of pages of poker literature.
The real answer has to do with the probability of what your opponent is holding, the probability of each of his actions (fold, call, raise), and the pot size.
Sometimes it’s not even wise to bet even if you know you’re holding the best hand at the moment. Sometimes it’s wise to bet even if you’re holding the worst hand at the moment.
Your question is just way too involved to answer simply.
Well:
Ya gotta know when to hold 'em, know when to fold ‘em,
Know when to walk away, know when to run;
You don’t count your money when you’re sittin’ at the table -
There’ll be time enough for counting when the dealin’s done.
d&r
[Darth Vader]NOOOOOOOOOOOO!![/Vader]
You definitely count your money when you’re sittin’ at the table. Your relative stack size is vital knowledge to have, especially in no-limit holdem. You should always have an estimate of how much money you have and how much everyone else has. It affects play. I actually count my chips and the chips of everyone else at the table at least once every fifteen minutes. (And yes, of course I love the song.)
Other than that, I’ve heard of Doyle Brunson’s one word school of poker. That word? ATTACK. Aggressive play beats cautious play. Take that to heart. And I like to keep an ace of spades in my shoe for luck (not just in poker, but 24/7) and that’s worked pretty well for me. But, and this is vital, it must be in your right shoe. With the face pointed upwards. Don’t make an amateur mistake. 
Don’t listen to this nonsense. Everyone keeps an Ace of spades. Last week I was involved in a showdown with 5 aces of spades (one guy had pocket aces suited). Go for the ace of cups.
I’ve only been playing for 8 months, and I still struggle with betting issues myself. However, something I’ve heard several times is that when you call there’s only one way to win the pot (with the best hand), but when you bet/raise there are two ways to win it (with the best hand or by making everyone else fold). I’m not an aggressive player by nature, but it’s helped me to try to keep that in mind. 
This is a hilariously complex question. The basic answer is that there are MANY different reasons why you might wish to bet in many different situations. And of course, limit and no-limit are almost completely different in all ways. A few different reasons to bet:
(1) To make everyone else fold
(2) Because you are pretty sure you have the best hand
(3) To put more money in the pot, so that it’s larger, and if you win it later, you’ll win more money
(4) To make some people, but not everyone, fold, because your hand is one that’s easier to play against fewer people
(5) To scare everyone into thinking your hand is good, so that no one will bet on the turn, and you’ll get to see the river and maybe make your hand
(6) To make your opponent think that you are doing one of (1-5) and thus misplay