First you will need to generate a list of ideas, here are some techniques on how to generate them:

• What are you good at
• Ask your friends and family what they think you are good at
• Have a brainstorm session with a friend
• Listen about other peoples problems / frustrations that they have and think of a way to fix them
• Ask people what they struggle with in their everyday life
• Try to think of a problem/frustration you face on a daily basis
• Look at existing industries/services and think about how to improve them
• Look trough various website/forums/social media groups where people discuss ideas

Now that you have a list of ideas it's time to filter them out. Let's look at the demand matrix below

Our ideal quadrant would be the "high price with many customers", others are fine too but it will be harder to make it profitable. Avoid the "Low price few customers" quadrant because that is an undesirable product for the market that no one will buy.

Once you selected your idea you should research your competitors and  deliver a higher value product/service than they do and you will be on a good way to a successful business. If you have no competitors then you should execute your idea as fast as possible before someone else does it before you.