Basically you put your opponent on a hand, an over pocket pair, two pair, a straight draw, a flush draw, but generally when calculating odds, you want your opponent to be on a made hand and you want to be drawing against them. Suppose you have Ts9s on a board that reads AsTc7s, and your opponent raised preflop, and has bet out on the action on the flop.
You put him on AK. Provided you are right, you have 2 T's to make trips, 3x 9's to make two pair, and 9 spades to make the better flush. If an 8, 6, or J comes you also pick up an out on the turn to make a straight (the ideal straight card for you would be the 8 on the turn, however be careful of getting priced in. As a rule of thumb you should know better than to chase inside straights or draw to runners).
So you count up your outs (2 Ts, 3 9s, 9 spades, or 2+3+9=14), and consider the chances of your card coming out on the turn. (As a general rule of thumb, I use a switch no larger than the width of my thumb... But also you can multiply the number of outs you have by 2 to get your percentage. However the correct math is available on the forum, which involves number of known cards, number of known outs, but overall you're rounding and simplifying large choppy numbers. I'd give myself about 30% to improve on the turn in this case, and be likely to call anything but a huge overbet. If your opponent bets the pot, you are getting 2:1 on your money. But since he has a pair of aces and probably thinks it's best, he's not likely to bet the pot, since he is going to call. He will probably bet less than the pot. Look at his bet, compare it to the size of the pot.
Ideally, you want what you put into the pot to be comparable to your chances of winning, or less. However, keep in mind that if your spade comes, or your 9 comes, he's likely to bet again. So you have to take into account that there will be a larger bet on the turn. Or another bet for value if he thinks the card doesn't help you, like the 9 for instance. But if the T hits, he might not bet again, especially if he puts you on the T.
So you have to keep in mind that if you made your hand, you're likely to get more money. These go into two different catagories, expressed and implied odds, and really go into the game of drawing in hold'em.
And it's not within the scope of my understanding of poker math to go too indepth. I could discuss outs and drawing with you for hours, but you should probably pick up a good poker book instead.
Generally speaking I would call just about any bet in the exampel given with a deep stack against another deep stack, or in a multi-way pot. (Especially then, becuase your odds, both expressed and implied, are so much better).
Also, you have to take into consideration that 1: you may have less outs than you counted. 2: You maybe have more outs than you counted, and 3: you may be ahead. Also, you may be drawing dead.
I think that answers your question.