How do I know when to use tan instead of sin or cos? When I think I've got it right by using sin or cos, I've really got it wrong because I

I assume you are talking about on a right angled triangle and it sounds like you are having problems labeling the sides

The Hypotenuse is the longest side and it is opposite the right angle.

The opposite side is opposite the angle that you are dealing with.

The adjacent is the other one (it is one of the arms of the angle - adjacent means touching)

So the first thing you must do is put these labels on your triangle.

Now, which ratio do you choose???

Just remember this

Some Old Hags           Sin=Opp/Hyp

Can't Always Hide       Cos=adj/hyp

Their Old Age              Tan = opp/Adj

You usually have two sides OR one and an unknown one so you choose the ratio that uses these sides. 

I really hope that this helps