Thank You Page Builders Mentioned:
Lead Pages
Instapage
Ontrapages

Steps to Take:
Have an opt-in on your site already?
Go opt into one of your sign up forms on your website so you can see what your Thank You Page is right now.
How can you improve it?

Aren’t yet giving anything away on your website?
Use this Lead Magnet Worksheet to come up with something THEN build your thank you page.