No worries. The things most healers don't realize is that any healing that brings a target over 100% generates threat equal to twice the amount of overhealing. There's not really an easy way to estimate the amount of threat that this does on any given occasion, but the easiest thing to note is that there is an available agro meter while healing, and you've probably seen it. The green circle, upward yellow triangle, downward orange triangle, and red square. Just be aware that you shouldn't surpass the upward yellow triangle while healing, or you risk ripping agro of the tank.

As far as WHMs go, your general go-to should be Cure I, and sparing use of Cure II when and if it's needed. If you've got a Freecure proc, use it before it expires, as it is free healing. Here's a guide I wrote for a friend.