don't understand shouting. any tips of which ones are the best and when to use them?

Just open your mouth and scream really loud. Sorry I couldnt resist.

Unrelenting force is awesome. As is Become Etheral, Marked for Death, and Storm Call. Everyone is different with shouting. You'll get alot of mixed reviews on this topic. If you havent already, you should pay the Greybeards a visit. They're your Zen Masters when it comes to shouting. They'll help you hon your skills.

To add to Dovah's info, different quest lines in the game will provide access to different shouts. I'm not very quick on remembering which ones give you which shout as I tend to just play. But I agree with DD about Unrelenting Force. That is one of the earliest ones and is most helpful in tight spots. All of them have a place, some may just suit your style better than others.

Unrelenting force with all three words is just as powerful at level 8 as it is at level 80. In my opinion, thats the single most powerful most useful shout in the game and definately a lifesaver. Even if you have no intention of doing the main quest its totally worth starting it just to get access to that shout.

Example, you get surrounded by a group of bandits or draugr deathlords, just Fus Ro Dah them and watch te bodies fly back while you flee or plan your next move.