Question on new bike assembly
This depends how much it was assembled before going into the box. I've had one bike that just needed the stem bolts to be loosened and the stem rotated, plus the pedals fitted (using tools provided). The one I bought recently needed a little more work but still only using tools you'd carry on the road plus a pedal spanner. That was a used bike, and had been dismantled just enough to go back in the box.

The seller should be able to tell you exactly what is required. It's well worth asking them. 

Important things to be aware of:


Some bolts need to be done up to a specific torque.  On many bikes this can be quite roughly approximated but on carbon fibre and light alloy you shoudl really get a small adjustable torque wrench (sometimes sold as a torque screwdriver).  The parts should normally be marked when the torque matters.
The left pedal has a left-hand thread.  Double check that you've got the pedals the right way round, then grease the threads and do them up with your fingers for severalturns before finally tightening with a pedal spanner (a thin 15mm open-ended spanner)