Go back over your basics before you start into summoning, already knowing a fair deal about magic is one thing, but think of it as being the refresher classes at the beginning of a new semester.
Think about why you want to summon something. What do you want to accomplish? Why is summoning the best solution for that? What do you want to avoid? Go from there.
What's easiest to summon is going to depend somewhat on you. How you magic and what you focus on when you're working tend to specialize you some. For me, it's plant spirits, nothing big but functional around the garden and home. It could be very different for you. Think about what you usually work with and try to figure it out from there.