Learn French: Je veux devenir professeur. - I want to become a teacher.

When you have two verbs right next to each other like want and become, the first verb (veux) is conjugated to match the subject. The second verb (devenir) stays in its basic dictionary form, which is called the infinitive.

Where is the word for "a" before "teacher"?

In English we say "a teacher", but in French, you drop articles like un or une before professions when they follow verbs like être (to be) or devenir (to become). You just say the profession directly.

How do I pronounce the word veux?

The "x" at the end of veux is completely silent. You only pronounce the letters before it.