Working with Cubase

How can I convert a beat in Cubase?

What do you mean?

Do you mean 'record' a beat in Cubase or create a beat in Cubase?

if u have a track u want to save as a mp3 then u need to export(files -> export)

I want to record all my tracks in Cubase..

Since you never stated if these were midi or audio tracks, I'm going to assume this is midi. Whatever midi tracks you're using, just add the same amount of audio tracks to the grid. Then one by one record each midi tracks' audio to a audio track (remember audio tracks default to mono). If you give a brief scenario of your setup maybe someone can help you a little more in detail.