Find the weaknesses of the existing solutions and try to be better.
There is numerous examples of companies that did it. For example, Zoom was founded by the former VP of engineering from Webex because he saw issues in Webex.<p>Having competitors is a good signal as it means there is a market.<p>However, I would not go against a super huge player as they have a development team that'll likely "patch" the product and corrects the weaknesses.<p>Out of curiosity, what's your idea ?
As a developer you can use a theme for your initial version.<p>There are lots of reasonably priced frontend/backend themes out there that you can build on top of for your initial version.<p>Find a theme built on bootstrap or tailwind css both reliable frameworks.<p>Then you can focus on the app and ux to make a better solution.<p>You'll want to spend some time validating your idea, try to talk to existing users of the app.<p>Good luck.
It's OK if the App exists but try to differentiate on something other than pure design and UX. This is pretty hard to pull off first time, especially when you're not a really experienced designer.
You can do following things-
If you can improve the solution by X factor, do it.<p>If your solution is same as there's then price it down by Y dollar than your competition.<p>If competitor takes N seconds to provide solution, see if you can provide in less than N seconds.<p>If onboarding to competitor product is difficult, you can make your product onboarding simplistic and write better documentation then there's.
Talk to customers and potential customers of the existing solution, see if I can convince them to buy a prettier solution for more money, and when the answer turns out to be no (as it is in 99% of cases), move on to my next app idea.
Check the reviews to compile a list of things where you can be better. The App Store makes user feedback very simple for the app, but also potential competitors.