There's help all over this forum and then again through google.

I also recently bought my n900 second hand and even though I emailed the original owner to make a note of the security code, he wasn't forthcoming on that. My guess is he forgot it anyway.

After getting networking and ssh working, I decided to deal with the lock code and locked the device. Sure enough, the default didn't work. But after downloading JtR, it took less than 5 minutes to reset. Of course ymmv.