First, congratulations on your expansion and thank you for hiring another developer.
Focus on hiring the best programmer you can find as soon as possible. There is an exhaustive list of things you could do to help incorporate this new developer, but I think you risk delaying the hire. This person will need to spend time getting familiar with things, so why not involve him or her in the process? When you're part of the cleanup process, you know where you put things.
When you finally hire someone, try to keep track, organize and document everything you do to get this new dev up and running. This is the perfect time to discover what the new person needs to know. Examples.
- Getting a development machine configured.
- Diagram basic structure of the app
- Coding Standards (There's got to be something you do consistently.).
- Source Control
- Tests
- Planning and Estimation
You'll come up with more as the new programmer gets close to writing production code.
This way you don't waste too much time on things that aren't needed. It's tough to predict at first and we all know something will be left out.
Getting your second dev up to speed, may not be as smooth as you like, but don't sacrifice the hiring of the 3rd, 4th and 5th. If you need more programmers, you know your business is getting to a critical success point, so you want to make sure your team is ready to grow.