Work a little bit on the project every day. Consistency is key. Even if it just means developing for an hour or two after your day job. It sounds cliche, but 2 hours - 5 days a week for a year adds up to 520 hours you can build your product with. It's how my business got started.

Focus on building and releasing your minimum viable product as soon as possible. This will also help you gain some internal motivation. As soon as you have people using your product and start getting feedback, you'll feel that the project has a life of it's own and you'll just want to keep working on it more.

Have fun, and pick an idea that doesn't feel like work (too much) :)