Painting works exactly the same way as any other field of work. The more you do something the better you get at it. Your style will become more advanced and your methods will improve. But ideas? Where do we take those from? Well the answer will be different to every artist. If you are at the beginning of your painting experience then I suggest you try everything that interest you for example: still life, landscapes, portraits,flowers - whatever speaks to you first, What I mean by that? Well we all have intuition, that voice inside that tells us what is right and what is wrong. The same voice should suggest us what is it that we should paint. By trying different subject and media we are going to find out what do we like to paint the most and we are the best at.
After a few successes and failures we should start seeing a pattern. The subject that we get back to more often and the media of course. It should all happen completely naturally. Don't put pressure on yourself just paint, paint and paint. Yeah we should get the idea.
What about ideas later on? Where do we take them from? I would advise to take few smaller pieces of paper and just sketch what interest you and trust me there are things that pop in to your mind more often than others. Once you have few sketches look at them and think what could make them better. You shouldn't wait long for the answer. If you don't know search on internet for different methods, solutions, how to improve what you paint. You don't have to create everything from scratch by yourself. Its great if you can but painting or becoming an artist its a process. Learn from others. I am sure after a short period of time you may have so many ideas and not enough of time to execute them all. Choose the bests one and repeat the process until you create something you are very happy with. Then go "outside of the box" and think of few similar paintings, how could they complement each other. Think of the size, colour, anything that could allow you to take to the next level. I hope you found it helpful.
It would be interesting to know what it your way to find out what to paint next.