Watch tutorials and create, than practice, practice, practice. If you want a good design, you'll have to either invest your time and learn it yourself or invest your money (which equals time in the end ) and pay someone else to do it.
It took me about 30 hrs to make an UI into my game. It could've been much cheaper if I just paid someone to do it, but I don't regret it. I've learned something new which might come in handy in the future.
example:
- Google "photoshop ui tutorial"
- Ok, I want to use this one in my game
- after 30-60min, done
- If you practice like this every day, I'll call you next year with a job offer and you'll create something similar quality under 15min.
It's actually very easy to be good at graphic design, it just takes time....