WOW, that's truly amazing for a first quilt, mine was a cheater panel; but you should be very proud. I would pick one of the colors in the quilt, maybe a dark blue, is there some purple in there? I use Warm & Natural in my quilts, I like the way it hugs to the cotton top and backing, making it easier for me to smooth out the quilt top; but it's personal preference. An all over meandering might work or a stitch in the ditch, but with a small zig zag stitch or a curving stitch. Try a sample and see what you like, whatever you choose I'm sure your finished quilt will be beautiful.