I'm not sure if I will be able to help you. Art is just something you have to learn to do, honestly.
Do you have any experience using programs like Photoshop or similar? If not, that's where you should start.
I do have some other advice for you.
I suggest you wait with making the main menu. It's not going to be important for a while and it will probably just get in your way while you're working on the actual game. In my case, I think it was the last screen I added to my game.
When working on UI, I tend to not worry about the art at first. What is more important to me is to just get something on the screen and interacting with it. Layout and user interactivity is much, much more important.
But you asked about workflow, so here's my general workflow when making UI:
- First, I draw it out on paper. Nothing detailed. It's so I can get a more clear picture of what I'm making before I start making it, and maybe catch some problems I didn't think of at first.
- Then I do the layout in Unity. Sometimes with flat colors, sometimes with empty RectTransforms. If you haven't worked with Unity's UI system before, I strongly suggest you do a few tutorials.
- I may put in some placeholder graphics here, if necessary. It makes it easier to work on something if I can see it.
- Next, I will start coding up behaviors and logic. If I know two GameObjects need to talk to each other I'll prepare some scripts to do that.
- Unless the code is art dependent, I will try to finish as much of the functionality as possible. Make it work!
- Once I'm happy with the layout and the functionality, I start making it pretty, because now I know exactly what art I need to make.
I use Photoshop to create pretty much everything art-wise. I sometimes makes things from scratch, but I also use Google and Pinterest to look up pictures of things if I need to make an icon for an item or something. For instance, I wanted a traditional look for the UI of my game, so first thing I looked for wood and paper textures. Then I heavily processed them in Photoshop to fit my needs. If I can't make it work or I'm unhappy with the result, I start over.