Learning about shrubs

[GrowingAGreenThumb]

So I have a black beauty Sambucus, and it is flowering nicely (put in a couple of months ago. It is about three feet tall but only has three branches from the ground (sorry about the lingo, new at this). All three want to fall to the ground, but they are tied up in a prism configuration, but the bush looks incredibly bare in the center.
I read one site that says that I should "pinch" it if it is "leggy" to control "branching", and a whole bunch of other terms I don't know. Can someone point me in the right direction to research these terms, or explain how I can get this sad looking plant to look like a full bush?

Thanks for your patience so far

[bullthistle]

Generally pruning plants is done when they are dormant, even evergreens go dormant, and depending on what you are trying to do with the plant depends on how its pruned. If you desire branches trim back the branches by about a 1/3 and that should fill in the bottom, then the following year prune the branches that branch inward off and prune back the weakest branch of double branches.