I'm assuming that you are asking about free motion designs on a machine? Stencil choice is a personal preference thing for sure. I hardly ever use a stencil anymore, but if you are a beginner, look for shapes that are not too tight and where you will not have to backtrack much -- you might like something like a vine.
I don't use my machine's computer-driven designs for anything other than very small items -- just playing around on a hotpad, for example. I don't like them.
I free hand. I usually use landmarks from my block design to orient and then "doodle" my general design with occasional variations. I plan my quilting by taking a photograph of the entire quilt, printing it gray scale full sheet, and then playing around with a variety of potential quilt designs on paper before I begin.
The rule about starting in the center is more of a general guideline than a hard and fast rule. It's a good practice, and usually is the best option, but sometimes it makes more sense to start elsewhere. If you quilt is layered carefully and basted well (pin or glue or stitchng) then you aren't likely to have trouble even starting off-center.
I start the motif with a couple of stitches in place to secure the end. When I come to the end of a line of quilting, depending on a few different factors, I may use my machine's knot feature, I may leave the ends long and pull through and finish with a quilter's knot, or I may just snip.
I am very much of the "give it a try and see what works for you" school of quilting. Others may give you other approaches, but what it all boils down to is finding a balance of what you enjoy doing and what works well aesthetically.