If you want practical advice, I would say look at what you are making, find existing projects that have similar functionality to what you are trying to make, take lot's of screenshots of them, put them all in a folder then note what works and what doesn't, and use it to plan your design.
When you have a minimum viable product (hopefully you've planned a little bit so you don't have to restructure everything) put them side by side and pick your own work to bits.
It's easy to lose sight of what you are trying to do if you don't take breaks while designing especially if you are new to this. You can end up making something really weird if you don't take your time and try to push it all out in one go. Allow your brain time to digest what you are doing in in the background while you do other things.
Other helpful tips:
Combine actions as much as possible without being confusing. If you have a flow that is 3 or 4 pages/actions long for a mechanism that is relatively simple - you dun goof'd. I like to aim for one CTA per action but I will settle for two, three if the action is relatively complex (but all cases are different - there are no rules).
Spacing and consistency make things look polished - it's 'not important' as far as functionality goes but it goes a long way for making people want to use your product. It also gives subtle clues as to the pattern or template they should follow to get to the next action. As in: if the CTA button is usually at the bottom but then for some reason it shows up in the middle or the top you are breaking the 'shape' the user expects to encounter while progressing through your app or site or whatever.
Same for text entry fields, buttons, or text fields - unless you have a good reason to change their location, keep similar things in the similar locations, keep similar mechanisms the same size as each other and have everything act as consistently as you can.
Keep it simple. If you hand it to someone and they just freeze and ask what they should do, you need to go back and make it more clear.