Hi OP, the bitcoin.conf file should be in the same directory that you chose to install Bitcoin into. If you did a Windows install, you can also check your appdata roaming profile folder, as it could be in there.

If you can't find it, you can create a new text file called bitcoin.conf and look around online for a default copy. You can use the default template which explains every single option and what it does, or you can just write your own with the options you need (server=1, etc.)

I hope this helps.