Ideas
I've always had an abundance of ideas and never enough time to write all of them. It comes down to being aware of your world around you.
Copy the Master
In art, you will often do a master study. Where you take a painting, you admire and then try to recreate it. While doing this you'd learn how the composition works and the different techniques. You can do the same thing with writing. Take a story - preferably one that is out of copyright so if you do an awesome job you can still publish it - and you try to recreate it. Use the same structure but change an aspect like setting or the gender of the characters. I started by taking aspects that I liked. The talented character with terrible brothers from Anne McCaffrey's Harper's of Pern. I then put her into another situation and saw what I thought she would do. Robin Hobb, Andre Norton, and Mercedes Lackey were all my teachers. Usually, it was elements from the world or a side character I wished I knew more about. None of these got far, but they taught me a lot about character development and world building.
Observe the world
Just like I took elements from books you can do that with real life. A political figure, a hero, a random article from a magazine; any of these could yield up interesting characters. Some suggest writing down these characters in a notebook. I like to write a scene with the character. Sometimes I'll write a few scenes but in different worlds or situations. I try to keep them straight by giving them the same name across the stories.
What if
I'm a speculative fiction writer and the core of that genre is asking yourself WHAT IF? What if dragons were real? What if you could do magic? What if the stories we heard as kids are real? What if World War III started? All of these lead onto other questions and as you fill in the gaps your world is created. Some of these are wish fulfillment, after all, I always wished I had superpowers?
Events
I love history and the best part of it is to put yourself into the shoes of those who experienced the event. In particular, I wonder what the event would have been like if I was different people there. You can throw in some of the what if elements as well. Like an adjunct fighting at Waterloo but what if it was a woman dressed as a man because she is trying to save her brother.
Collect all of these ideas. Don't throw them away. I used to write scenes and just labeled them story 1 through to story 890. Yes, I literally have almost a thousand ideas for stories. Ironically, I hardly use them as I see them more as a study in character, plot and world building rather than seeds of novels.
Your homework is to collect ideas.
You can also use writing prompts and story generators. Below are some links to useful sites.