I'm assuming you are using windows media player (WMP) to rip it.
WMP uses WMA as it's default
Towards the top left will be ORGANIZE. Click on the little down arrow just next to it.
A little window pops up, at the bottom of which is OPTIONS. Click this.
A new window pops open with loads of tab options. Don't panic.
Click on the RIP MUSIC tab
Halfway down the screen now you can see RIP FORMAT and underneath that OPTIONS. It should say WINDOWS MEDIA AUDIO in a box with a dropdown arrow next to it.
Click the arrow and select MP3 from the menu that drops down.
Where it said WINDOWS MEDIA AUDIO, it should now say MP3
Click on APPLY, and then OK.