A bluff is a bet or raise with a hand that is unlikely to be the best hand, intending on making all your opponents fold. If it works out, it’s a nice thing. But: timing and dosis is very important here.
For you as a beginner, who is playing on the low levels where people are rather loose and call all the time, the rule should be that you should never try to bluff!
Elsewhere, you do not bluff if:

• You have multiple opponents – someone will like his hand and call your bluff
• When pots are extremely big (if the pot is 20 $ and it’s 1 $ to call, even players with weak hands get good pot odds and call)

If you suspect that your opponent is bluffing, consider this:

• If you have to invest 1 $ and the pot is 10 $, you only have to catch a bluff 2 out of 10 times and you win 20 $, but loose only 8 $ the rest of the time.
• If player A has bet, player B has called and you suspect that A is bluffing – fold! Player A maybe bluffing, but B cannot be bluffing, so unless you think you can beat B as well, a call does not make sense.

This is a bet or raise with a card that is likely not to be the best hand now, but still has a good chance (6 outs or more) to improve to be the best hand later. If your draw is completed, you have successfully filled the pot. If everybody folds, you win the pot right there and then.