if you are sincere:
learn about tawheed and how to avoid shirk -> learn how to pray -> jihad for the sake of Allah
(see kitaab at tawheed)
if you commit shirk and then die, all of your deeds will be for nothing and you'll abide in hell for an eternity. one of the sins Allah subhanahu wa ta'ala will not forgive is shirk.
the definition of shirk is to associate partners will Allah. It is very dangerous in the sense that it is very easy to fall into.