Newbie in need of advice

hello everyone my name is lauren im really new to the domain name world and imm afraid i dont know too much yet, althought im very excited about it and im reading as much as i can about it.

im just wondering what makes up a great domain name!

is it a .com or a .org or maybe even a .co.uk

is it good to have numbers?

are random letters a waste of time?

thanks for reading and sorry if it seem a silly question

lauren