Had lunch in French

Assuming you mean how to say "I had lunch" or something similar, you could say:

J'ai pris le déjeuner

J'ai déjeuné

These aren't the only ways, however.

For other viewpoints (he had lunch, you had lunch, etc) you would change only "j'ai", replacing it with:

You (informal and singular): Tu as

He/She: Il or Elle a

We: Nous avons

You (formal and/or plural): Vous avez

They: Ils (males or a mixed group) or Elles (females only) ont