You should really have a read through the Getting Started section, try here
Do you want something that is just going to sit on a shelf or you are going to try get approved in, there is a big difference between the 2. If it's just for a shelf that the Black Series is a nice display piece, if it's for 501st approval then you will have to spend a little more and do a little research.
Have a look through some of the build threads, will give you an idea of what is involved.
Also have a look at the CRL's (costume reference library) these are the costume requirements for joining the 501st, for Shock troopers look here