404 error. Cannot login to my admin

If you mean that you changed the site URL settings, see:

http://codex.wordpress.org/Changing_The_Site_URL

It’s actually not that hard, you just need to be careful and follow those instructions carefully.

But if you’re not comfortable doing so, you can post a job listing here:

http://jobs.wordpress.net/

Or perhaps your hosting company can help you.

These forums don’t provide hands-on support like that, sorry.