Before venturing into the world of Tamriel, it is important to choose the role you wish to play. There are three main roles in ESO. These are the damage dealer, tank and healer. Picking your role totally depends on your playstyle. It therefore is highly recommended to try all roles with your first character and then start a new character (or re-use the old one) to pick a role you wish to play.
After and during leveling your character to the maximum level (V16 at the time of writing) you will have to train your roleplay. Know all the strenghts and weaknesses of your character and his/her role. These strenghts and weaknesses can be used to make you play your role on its strenghts. Often guides will inform you only about the strenghts of the roles you can play. Weaknesses are of the same importance of your effectiveness during your adventures. Because knowing your weakness makes it easier to pick the best compensating group members. For example as a healer, your biggest weakness is dealing damage. Meaning that you need a well focussed damage dealing player to compensate this weakness. Creating a powerful synergy.
Playing and sticking to your role is very important to create this synergy. It is unnecessary to waste skillpoints and attribute points into your weaknesses. It is better to invest your points into your strenghts and make those strenghts even stronger. However this also means that your weaknesses will also be greater. For example a damage dealer, that does not compensate for the lack of Health, will become a glass cannon and needs support from a powerful healer to stay alive and be functional.
It is better to be efficient than effective. Because an efficiently played role will eventually lead to an effective one. An effective player will get the job done, but will have a lot of trouble doing so. Effective players often run out of resources quickly and are left doing nothing during fights. They often need a lot of support and are highly dependable on group members. An efficient player however knows how to manage their resources and time in such a way, that this player will (almost) never run out of resources, no matter the duration of a fight. This makes this player of great value and the backbone to a group.