I think joining writing competitions is a great way to practice your skills as a writer.
Have you come across the "Writing Prompts" subreddit on reddit.com (https://www.reddit.com/r/WritingPrompts/)? Basically, sort the view to "New" and when you see a writing prompt that interests you, write it, submit it, and see how well it does.
The votes do not translate to what's best, but it does give you an indication of what would be popular. You'll also get feedback from real people, which I think is a plus.
Anyways, wish you the best!
I am a writer as well. I am so happy that you want to reach out and get engaged in writing in a public way. One place that you may want to consider this website: https://nanowrimo.org/. This site is not necessarily a competition. It is about a community of writers supporting writers. You can get some challenges to work on and solicit feedback from others on your writing.
The most important thing in writing is to write all the time. Make sure you journal. Write when the mood hits you and when you are not in the mood. Be passionate about good writing. Learn from other writers by reading a lot.
Good luck on your writing journey.