As someone that's been a part-time content creator for 4 years (and full-time for a few weeks!) here are some suggestions to get you started. PLAN AND PREPARE!!! Figure out what content you want to produce (your niche), plan how often you will post it, and on what platforms, and create systems (like templates) to keep the train moving. I use an app called "UNUM" to plan my Instagram feed, and "Canva" for the templates on my second account @live.bsl.
Write down ideas the moment they pop into your head! If I don't write ideas down they're as good as gone. Take some time to "brain dump" all the content ideas you have into your notes which you can add to and structure over time.
Don't sleep on hashtags! They help you reach a new audience in your niche - make a list of go-to's in your notes and copy/paste.
TikTok and reels are currently the best ways to grow, without a doubt! Don't be shy!
How can you spread the same piece of content over multiple platforms?
Collaborate, collaborate, collaborate. Reach out to other creators and do what you do (or will learn to do) best, create!
Engage as much as possible. I reply to AS MANY comments and dm's as possible. This will go further than you know. When I first broke through on TikTok I would reply to every comment on viral videos (thousands of comments sometimes).
Last but not least, put your ego aside and learn to not care about what other people think. People will discourage you, make fun of you, discredit you... you have to have thick skin. Your family, friends, co-workers, will have something to say, don't let it stop you. The best content always comes with the most opinions, so take risks and learn to live with what people say!