1. If you're a minor, don't tell people that online. Ever. Either lie about your age or don't mention it. You make yourself a target. You are not immune (and neither was I).
2. Only use your real first name online if it's a common name. (And even then, consider long and hard)
If you have an uncommon name and you share it, it's easy to find you. Example: when one of my friends said she was going to kill herself, I found her home phone number and address with only her first name and general location in under five minutes. (I got to her family in time)
3. Do not share where you live. Even if you live in the biggest, most populous city in the world. Don't make it easier to find you.
4. If you must share your age or a general range, do not share your birthday. If people know you're 24 and know your birthday is January 10th, then they know your birthday is January 10th, 2001. There is so much damage someone can do with that information.
(If you have a relatively uncommon name, then even "20s" could be enough to find you with your birthday).
If you want to celebrate it, choose a random day every year.
5. Don't mention local events (political or fun (ie a fair)), landmarks, or post photos from your area.
Watch any of the professional geoguessr players if you want to see how dangerous a single photo can be.
6. Don't share photos of anything distinct like your face, tattoos, unique jewelry, etc. This is to keep people from recognizing you in the wild.
And finally, 7. Don't share your address or phone number. Period. Ever. Not your Snapchat or WhatsApp, not your facebook or any place that uses your location or phone number.
It is so easy to find people. Please be careful to not be found. All it takes is pissing one person off to get doxxed.