To begin with use the offline version of ARK: Survival Evolved to prepare yourself for when you eventaully feel ready to play online. It is advised that before you attempt to play the game online you wait until you have reached level 20 and have a firm grasp on the controls and gameplay.
Online mode places you in an island server inhabited by the same dinosaurs you encountered in offline mode but now also with other players. How you decide to interact with these other players is entirely up to you. It is advised though that you try and team up with them so you create a tribe that works together.
Working together with other players will greatly improve your chances of survival and has many benefits including enabling you to share resources with them and plan out attacks on bigger targets that you would not be able to do if you were by yourself.