I’ll be in the residential or partial hospitization unit… finding out tomorrw. Any tips for how to gain weight or any general tips?

Focus on recovery. Even when it gets scary and you start to gain some weight, keep reminding yourself that it is healthy for you to do so, that you are there to heal (both mentally and physically) and that only you can make the decision to enter recovery.
I applaud your strength. Admitting to a problem and entering treatment is one of the hardest parts.