Every time you going into the Internet you going outdoors.

Have you ever thought about this? This is mad may be, but listen:
Even when you are sitting at your safe home, in your favorite chair, at moment you turn on your smartphone, or laptop, or anything with the Internet - you just went outside. Of course all of us have an illusion of safety, but in reality we are walking on the streets, where sites are houses and districts. May be not to many people know who you really are, or how you looks like, but they are with you.
Remember about it, when you will upload your photos. Be naked in the Internet is the same, as go naked through the streets. And nowadays it isn't true that people from Internet don't know who you are because of socializing services, search engines. Think of how you'll feel, if some hundred guys will gap at you naked in reality. It is the same on the Internet, just people have an illusion that no one know.
Remember it when you writing. It's the same as post about yourself on city's newspaper. If you abuse - it's the same as you abusing on the streets. With small difference: it would be remembered.
You can't erase most of the data you have uploaded - even if a site says, that all data have been erased it isn't so usually. Facebook have more then 150 pages of personal information about you, every site with the "like" button take a date that you have been there, and what you were doing here. If you logged out - it will take this information with probability to your account (all stuff has it's number, you usually come from the same devices, and it isn't difficult to know that most possibly that was you). Google will never erase all history of search - his search engine was built this way.
You shouldn't be relaxed here, you didn't know who lives on your district. You are under attack at most of the places by advertising and temptations to do, what you really don't need to. They are playing on your base feelings. And they don't care about you, about your life, your feelings.

If you have relations with your friends - remember, that it's very difficult with time to find a border where they are real, and where they are looking as they want to, or fear to, or forced to.
Remember, that in offline life, when you are going for a walk with your new friends you have a time. Time to think, to go back, if they are going to place where you don't want to go, or doing things that you hate. And you have no that time on Internet - just few clicks between you and your friends favorite strip-bar.

If it will be easier for you to remember - you can think, that in the moment you come online, you just arrived to large city, unsafe and unpleasant, which needs you not more then a part of him. You would be watched in it all over the time. Whatever you do. Wherever you go. Whoever you talk. And it will not forget.