Why does my Pokemon Go app close back to homescreen every time I select a starter?

If your phone might not have enough RAM then try to limit the RAM being used for other apps. You didn't say what kind of phone you have, but if it's a smart phone, then you should have a way to view what processes are running and close everything that's not needed. There are also apps for Android (not sure about Apple) that will make this easier and also show how much RAM you have available. Restarting your phone right before playing might also help.

Unfortunately, Pokemon Go just doesn't work on some phones right now, even when they meet the technical requirements. You may have to wait for Niantic to fix some of their issues or get a new phone to play.