How can I help with the situation in Houston?

I'd contact someone at TR.
https://www.rallypoint.com/units/team-rubicon

I just got back from the George R. Brown Convention center (10k evacuees). There is plenty of food, clothes and blankets. The one thing that is going to be running low is blood. Our Blood Bank is getting no donations from the area, because all the donors are either evacuated or working to clean up. They are getting blood from other centers around the US.

If you are active duty then I would talk to your chain of command and state your interest. If you're a veteran there are plenty of crews coming here from all parts of the nation to help with the rebuilding of the lower parts of Texas.

Thank you in your interest to help. We in Houston appreciate your offer to come down. You well be treated well and very much appreciated.