I've seen many people who think they need to choose a line and stick with it like in other MMOs. Doing so will probably gimp you in ESO.

Firstly, by having a single skill from a class line on your bar the skill line will level whenever you gain regular experience. You can also unlock later skills in a tree without unlocking previous ones as long as the tree is the appropriate level. Finally, you will have easily enough skill points to get every class skill so you won't really loose anything by taking situational skills from your less preferred lines.