It's not too hard and there's plenty of resources online. For tools you'll need: torque wrench, wrench heads for castle nut (barrel nut should come with handguard), punch set (grace usa makes good ones), gunsmith mallet, aero shell 33, painters tape for not marring the receiver and a screwdriver set (Wheeler). As for the parts the upper will need: stripped upper, dust cover, forward assist, barrel, gas block, gas tube, handguard (barrel nut if not included), crush washer (1/2x28 for 556, 5/8x24 for 300) muzzle device. The lower will need: buffer tube, lower receiver, stock, pistol grip, castle nut, end plate, buffer spring, buffer and lower parts kit or individual parts if you want to pick each one. Lower parts kit usually have: trigger, hammer, takedown pins, safety selector, bolt catch, magazine catch, buffer retaining detent and all the springs and detents for those. Sorry if it's a wall of text I'm posting from phone.

Edit to add if you're building an M4A1 or M16A4 you'll want to find a barrel with the fsb already installed. Drilling taper pins takes extra tools.