Don't despair OP. As of next patch, every tree's spammable skill will be the first unlocked, both for ease and to make it more obvious. They also currently have a small crossed swords /dps icon on the skill description while in the menus in order to guide you further.
I just started rebuilding my high elf magsorc to be decent. Never tried to make her a powerhouse. She is cp 485 now. 5 piece necropotence, and my spam ability is crystal fragments. With buffs from gear sets and using fortifying meal, i can stay back and let my pets fight up close while i crystal fragment non stop. My magika regen keeps by bar at 40 % + full continualy. Along with my ultimate it makes very quick work of delve bosses and mobs. I know i still have a very long way to having a Good character, but its a start. I still have alot to learn, and only fair at world bosses. Need to work on using potions and other buffs. And i need to make my gear better. Learning how to animation cancel, still not good at it. But being able to spam crystal fragments without running out of magika does a heck of alot of damage.
And sometimes i even use one of my staffs. Lol
Spamming ability is some thing you'd use after dropping your dots ( damage over time ) abilities to fill in your dps are instant and have low cost for example