Assembling a bike is just like anything else - if you know what you're doing, its easy.
If you don't know bikes specifically but do know mechanics generally it's reasonably doable.
If you're equally inexperienced in both general mechanics and bike mechanics it can be a nightmare ending in ruined parts.
My advice for you to do your homework first, there's
www.sheldonbrown.com,
www.parktool.com and www. bicycletutor.com that all offer decent-to-excellent advice and walk-throughs for working on your bike.
The come back here with pics and specific questions.