Most of this is advice from someone who had an iPod and then an iPhone -
Be wary of the ones disguised as calculators cause when I was a teen (2015 or so) those came out in the news as being hidden apps and then my mom started checking my (real) calculator app
Ive been using private tumblr blogs (password locked, only restricted to people logged in on tumblr who have my blog password) to store photos, diary entries, important info. Google apps are fine but be extremely careful and be aware where/if they sync to anywhere else.
If you have an iPhone and share an iCloud backup with your family, turn off iCloud sync to avoid any accidental sharing of photos/other things you don’t want shared
Most text tracking software does not know how to work with free texting apps over WiFi - I used textfree as a teen but there’s plenty of others.
Make accounts your parents don’t know about. I have a second email where I take care of riskier things and a first email they used to check that has nothing suspicious. Email is a big one you’ll need esp as you get older.
Seconding the keep things in code - a friend of mine used to text a single period to indicate that I shouldn’t respond or acknowledge anything until she got back - this is good to prevent the accidental “I deleted my messages but then they responded”
When in doubt, go old fashioned and niche. Keep important things on flash drives and keep those flash drives hidden. Older versions of apps and iOS software are less easily restricted and tracked by software because most tracking software came in later versions and become more efficient with every version (ex my mom blocked tumblr.com on my phone but an older iOS would still allow me to visit some individual blogs whereas later versions blocked every blog). Use browsers and search engines that aren’t mainstream - I used to use Bing quite a bit. DuckDuckGo is pretty good but catching on in popularity, I used to use Ruby and IE. At my job last summer we had a niche version of Google Apps by another company (we weren’t allowed to use google since it owned the data - I can’t remember the name but hardly anyone would think to look there). Cisco Jabber was also a good IM app at the office but can be used for personal use as well.
As technology advances, people will be expecting you to keep trying to hide things using the latest and greatest tech. All tracking software will be updated to try and keep up. People forget about old versions and assume that no one wants to use them anymore. Use this to your advantage. People don’t know about the more niche versions of things, or don’t think you’ll use them, or just don’t think to check them. Use the tech that falls through the cracks.
Most importantly, be safe and careful. I lost years worth of things that were important to me when my parents went through my phone one day - if I’d been more careful about deleting things, I would’ve been in far less trouble. It‘s painful to lose memories, connections, any of these things you store privately. But the most important thing while you live with them is your safety. The things you cherish will never really be lost, no matter what your parents do to your phone or whatever steps you have to take (or things you have to delete) to protect yourself.
It does get better. I spent my teen years afraid for my life but now I’m in college and have privacy that I never dreamed of having. You will make it out. You can do it