Did anyone mention crimping? Yes, it's that important. Find and buy a
ratcher-type crimper for solderless terminals, it should cost around $50, and take all the non-ratcheting type and throw them away.
Tools, like Al mentioned.
Get some wire of different sizes. Get
electronic type wire - stranded with relatively fine strands, stay AWAY form electrical (house wiring - THHN, THHW, etc) type of wire, it is way too stiff. #20 for sensors, #14 for light power, #12 or #10 for big motors, red and black at least.
Practice wiring stuff with crimp terminals and
screw-type barrier blocks , some switches and light bulbs or motors and batteries.
Learn why you want the thickest wire you can get (within weight limits) for high-current applications.
As someone mentioned, read last year's rules and electrical guide.
Look at some photos of neatly wired robots. Or go to an off-season competition and look. Ask a lot of questions too.
Don
NOTE: The links are examples,
DigiKey is not the least expensive supplier around.