And no worries about breaking anything, as long as you keep some simple things in mind:
Handle your components with care, especially when it comes to contacts/pins. Especially the CPU will need absolutely no force to get on the socket. If you feel you need to use force, something's wrong. A CPU just falls into place. On the other hand, do not be afraid of using force to plug in the power cables in the motherboard and seat the RAM. Fasten screws tightly, but do not overdo it. After too tight comes too loose (screws or threads break). Always work on a table, wooden floor or similar, never on the carpet, as you do not want static discharges to ruin your components.
You should always start with a breadboard setup, so you can easily find out, whether the machine works or not, without having to take everything out of the case again. Chances of breaking something vital are much higher, if you have to disassemble the whole thing repeatedly.
And, most importantly and as was mentioned before: Do not rush! Everybody wants to get their new PC going as fast as possible, but chances are, if you rush things, it will take you two days longer, because you missed something, etc. and have to find and fix the mistake.
P.S.: And when you finally put the motherboard into the case, don't forget the stand-offs and make sure that you only have those stand-offs in the case that you need.