Any tips on writing up my thank you speech?

Start with thanking everyone as a whole (like Hank said, you can't thank everyone), those who couldn't make it, those who came from far away. Then thank your vendors. Move on to wedding party and MCs (if you have them) or anyone who particularly helped you get to that day. Finally move on to family... Siblings and then parents (relationship dependent obviously). If you didn't do your own vows, this is when you want to talk to each other (this should be the longest part then). Ours was close to 10 minutes and that's what we did. You don't want to go longer than that! The worst speeches I've heard have inside jokes that no one else can understand or are completely surface level - I've actually been to a wedding where the only things the groom said about his new wife were she checked off the things on his checklist (he was specific). Speak from the heart and it'll be great!

I would keep it to the point. Depending on the size of your wedding, it might be hard to thank everyone individually (I had around 60 and tried, thinking it was a small enough group to give a personalized thank you but ended up rambling quite a bit). Just like how you wouldn't want other people's speeches to run on forever, keeping yours tight will be more engaging.