Depending on your expected lifecycle of the app, you should use an adapter between the DB and the rest of your app.

Have an interface made up of clear methods which you do all your business through. That way, if you want to change the DB, you only need to rewrite the adapter.

I believe SQLite is a great, low-insallation-needs portable DB you can start your project with. If it turns out your startup gets millions of users, you can afford to switch.