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C-c C-m, or simply C-c RET will give you a prompt that asks you for a LaTeX macro. You can use TAB for completion, or the up/down arrow keys (or M-p and M-n) to browse the command history. In many cases, AUCTeX knows which arguments a macro needs and will ask you for that. It even can differentiate between mandatory and optional arguments--for details, see 5.2 Completion.
An additional help for inserting macros is provided by the possibility to complete macros right in the buffer. With point at the end of a partially written macro, you can complete it by typing M-TAB.