It's not like putting a bike together ;-)
- but - you will get a lot of satisfaction and probably learn a lot if you do it yourself.
You need to have a good idea of what your are doing and be able to work with TLC and no brute force.
You need some knowledge about electrics.
Have a read here, click here
but note there are several other sites, just Google "how to build your own computer" without the quotes.
DO ensure all the bits you assemble before building are compatible - this is vital to the end product.