Fairplay to you for joining up, Hive is an amazing place, and kudos on trying to hone your creative writing skills, there are plenty of places here centred around writing so I think you'll find exactly what you're looking for, I'm sure your mum will help you discover all the cool places Hive has to offer.
The World Building Community - Here's a great place for world-building, everyone who posts here loves it and each is a well of information concerning the subject, they also release a lot of prompts to help get you started if you haven't got an idea to work with. The prompts are great, and the head of this community is really passionate about World Building.
Scholar and Scribe - Here is a really good writing community, where you can post a plethora of different content, from poems to short stories, and they accept serialised content, so if you were thinking of releasing the story you're thinking about now as a multi-part series it's well worth checking out.
The Ink Well - Here's another great writing community, who are quick with feedback on work. The rules are that you have to read and comment on two posts for every one you publish there, which is good because it gets everyone reading each other's work. I imagine it will be helpful for you to check this place out, I've gotten a lot of good critiques from the creators there which has helped me improve. One thing though, it's only for short stand-alone stories, so keep multi-part creations for the other communities.
I love Sci-Fi, so would be interested in reading your work when you start posting it.
Good luck, and welcome aboard!