Do you need a teacher to become a Buddhist?

Yes.  Absolutely.  The Buddha.

For the Blessed One was the arouser of the unarisen path, the begetter of the unbegotten path, the expounder of the unexpounded path, the knower of the path, the expert with regard to the path, adept at the path. And now his disciples follow the path and become endowed with it after him.

Ven. Ananda, MN 108, Gopaka Moggallana Sutta

Many people are familiar with the Buddha’s statement that spuritual friendship is not half,  but the entire holy life.  People often miss the way he explained it in detail:

And through this line of reasoning one may know how admirable friendship, admirable companionship, admirable camaraderie is actually the whole of the holy life: It is in dependence on me as an admirable friend that beings subject to birth have gained release from birth, that beings subject to aging have gained release from aging, that beings subject to death have gained release from death, that beings subject to sorrow, lamentation, pain, distress, & despair have gained release from sorrow, lamentation, pain, distress, and despair. 

SN 45.2, Upaddha Sutta: Half

We are all extremely lucky to be alive in a time when the Buddha’s teachings are still available to us. We do need the help of many spiritual friends.  But the Buddha is the only teacher required.