Was Homelander the first Supe?

He's probably not the first, but he's likely one of the first, and almost certainly the first one that is still alive today.

Comic book answer: The first Supe was called Stormfront. He was created in Nazi laboratory by Dr Vogelbaum and his DNA was used as blueprint Vought improved on when creating Homelander. Dr. Vogelbaum was forced to create Supes for Vought as a condition so he can run away from war and survive. Dr Vogelbaum is the original creator of compound V.

He took Stormfront (who was a member of Hitler Youth then) with him when he ran away. It was near the end of World War II. The previous heroes that were created before Homelander by Vought experimenting with injecting Compound V into people are Team Payback. The original incarnation of the team were all killed, along with the soldiers they were supposed to aid, in the sole military operation they took part in (The Boys, Issue 52 and Issue 53). They used powers unwisely in the middle of the war. Vought tried to sell them to military as the next superweapon but were forced to go back to the drawing board after the failure. They worked on improving the formula after that and spent billions and trillions of dollars on it, before finally creating Homelander and his team.

I don't know about tv show answer.

From the comics: No, Homelander was not the first superhero.

The first successful superhuman was Stormfront, a member of the Hitler Youth, when Vogelbaum worked with the Nazis.

I believe that Stormfront was the source of Homelander's DNA though.