For designing characters, a lot of people do brainstorming by doing a lot of silhouettes. I really like Feng Zhu, here you can see this process in action:
https://www.youtube.com/watch?v=4yKxY0KKrakIt's similar for other things (cars, spaceships, machinery, locations). You need to spend some time looking at references online. So if you want to draw a tank, you would search tank in google images. And maybe sci-fi tank or ww2 tanks if you want to go more specific. So that will give you ideas with which to come up with your own design.
Coming up with things in general is concept art and other videos by Feng Zhu go over this area significantly. It's not pixel art specific, but don't let that stop you.
Otherwise ask your questions a bit more specifically.