needing some help :D

I'm going to buy GW today and was wondering do I need to buy expansion too or can I play without expansion ?

Every game can be played seperately, but some games are better for beginning in GW then others(except GW EotN).
Personally i think you'd best start with either prophecies or nightfall, both have a good story line and a long gameplay.
You might want to start with one, and if you like the game buy the others ^^.

Prophecies, Factions and Nightfall can be played on their own, but can be combined to allow characters from one chapter to play content and use skills for another. Eye of the North requires you to have at least one of the other three chapters

Like the above players said you only need another campaign for eotn.However imo it is best to start with prophecies.Many important skills are prophecies only and it has the best storyline (imo) also there is a proph + eotn pack so you can get them cheaper.May i note that it might be a good idea to buy the complete collection pack when it comes out ?