How to decide my UI design if I am not a designer?

Find a page or a web site you like and borrow their ideas.

This is how you win-

Step 1 - Identify a list of designers that you think do really good work.

Step 2 - Know exactly what you want them to do, as if you're going to be hiring an expensive firm to do the work....Project Brief, Scope of Work, anything that makes their project deliverables crystal clear. They need to know what you will consider to be a successful project.

Step 3 - Determine what the market value of the project would be and come up with the equivalent development work you are willing to trade in exchange for them to complete this work for you. If the project is a $3k job, you offer $3k of your services to start but be prepared to do whatever they want. You can probably do whatever they need a lot faster than they think it will take anyway. Do not be petty here, don't nickel and dime them. You can be realistic but be optimistic. If your time is that valuable, hire a junior guy to do the work for you and pay him peanuts while you oversee it. The junior guy builds his resume while being mentored by you, while you make money. Win win.

Step 4 - Approach the list of designers from Step 1 and make them an offer. Be straight up with them and send them your project brief, SOW, etc. so they know exactly what you're looking for. Then tell them you can't afford to pay but that you are a talented developer (show them projects you've done) and ask them if they need any work done. Be open minded and willing to do anything in exchange for their expertise....Remember, they are doing you the favor, not the other way around so you should be willing to do anything.

If you selected senior level high quality people at Step 1, and did a good job of clearly explaining what you want, you will have put your best foot forward and will now have a top quality site/app. High quality people always deliver high quality work...Even if you fall short on your end, they overcompensate for you because high quality people care a lot about everything.

Make sure you crush the project they give you. Impress the hell out of them with your honesty, development speed, communication skills, best attitude ever, kill them with kindness, you are the nicest, smartest, fastest developer they've ever met.

You will now have a goto company/person for all your design needs and you will also have a paying client. The more generous you are with your time and effort, the more likely they are to hire you for paid work and the more available they will be to get your projects done. Your job is to impress them so much that they will always answer your calls and be willing to do whatever you want. But it all starts with you. You have to be generous enough that they are indebted to you and grateful to even have access to your time. You should always maintain a score where they always owe you more than you owe them.

If you select high quality people from the start, always have crystal clear project deliverables, make sure you all agree on the timelines, monies, responsibilities, always respond quickly to their calls or emails, you will never get screwed because good people always treat others fairly.

It's that easy.

Use a CSS framework like Bulma or Bootstrap. They have already designed components that you can basically just plug and play.

You can also use a template as a starting point as well:

For Bulma, check out https://bulmatemplates.github.io/bulma-templates/.
For Bootstrap, check out https://startbootstrap.com/themes/, https://themes.getbootstrap.com/, or https://bootstrapmade.com/.

So... I don't know if you've considered this, but build something without an GUI. Do something that's command-line, like developer tools, assembly pipeline, etc. There are tons of products out there that doesn't have a graphic interface and still does well.

Being a developer and a designer in training I second Refactoring UI (the book and videos are really helpful and aimed at developers).

In the past, I have used Bootstrap plus a theme from WrapBoostrap but I have always hated working with them both down the line.

The last product I have built (for my current employer) but all future products will be going this way is using Tailwind.css https://tailwindcss.com/, maybe Tailwind UI https://www.tailwindui.com/ when that gets released. And inspiration from Refactoring UI and certain sites like Stripe which I think have particularly nailed UI.

Could just pick a lib like bootstrap and buy a template.

It depends on your product, and many other things.