My name is Patrick I play tenor trombone, bass trombone and baritone.
I would love to start playing french horn.
Do you guys have any tips for me.
I have acces at school to double horn.
I need help to get started.

The big difference you'll notice is the mouthpiece size. It's like 20 times smaller (I recently started trombone and it's like 20 times bigger).

Since the harmonics on a french horn are so close in its usual playing range, you have to really be able to place pitch. I've found that lip slurs and arpeggios and that type of stuff are good for helping with this.