how to copyright my comic strip?

You don't need to. Creating a piece of media automatically confers you ownership of it's copyright. You can register it if you think that at some point the IP might be contested, but you don't have to. Poor mans copyright, as suggested by Kitty, hold's no legal value in court.

Kitty is wrong, the 'poor man's copyright' where you mail it to yourself never stood up in court, you could never prove that you didn't steam it open.

Jesse is right - soon as you finish the comic strip and attach your name and the C symbol, it's automatically copyrighted to you. To make it official for holding up in court you'd need to file with the US Copyright office, but it's best if you do an entire year's worth at one time, for the $40 fee.

put in in an envelope lick it and close it then send it in the mail to your self and never open it but u half to pay for it