This is a broad question, which would really benefit from a much deeper understanding of where you are at and who you are as a professional, but here are a few tips:

#1
A good idea will solve a problem. You want to be building a pain killer, as opposed to a vitamin.

#2
Once you feel like you have a problem to solve, you should validate your idea. Here are some simple tips for validating your idea: http://www.startuprevival.com/7-ways-to-validate-your-idea/

I also recommend entrepreneurs to test the market by setting up a landing page and sending some traffic to it. Unbounce is a great tool for building landing pages.

With a modest budget (just a few hundred $) you can drive traffic to your landing page through Google Adwords. Try and get people to fill out a lead form or sign up for a trial of your product. If you get some leads, it will help you understand if there is demand for your idea. If you don't, it could mean there isn't a strong demand.

You could also buy some market research data using a tool like Survata.

#3
Once you have a validated idea that solves a problem, make sure it is something that you have 1. an interest in and are passionate about, 2. you could execute on with your skill set.

There are a lot of great ideas out there that solve problems. But it's important to make sure it's the right fit for you.

Best of luck!