If you're new to sewing, I'd sugest finding and modifying a pattern. You could take a plain tank top pattern and add the collar, or find a pattern with a mandarin collar and modify that. You could even take a turleneck type pattern and leave off the sleeves and make the collar narrower.

If I can ever find my camera though, I'll try sketching what the pattern might look like.
EDIT: I found my camera.

I dunno how legible the text in the picture is (lol) so I'll translate:
The first thing you need to do is take your measurements. Take from the curve of your next (where you want the collar to be) to the edge of your shoulder, from your collar to wherever you want the shirt to fall, from your underarm to your waist, from your underarm to where you want the bottom to be, around the bottom, and across the width of your neck. Not all of these are shown in the picture, so don't freak out. I'll explain the ones that aren't shown: the neck measurement, you'll just use to mark the width of the neck opening. you need both the measurement from underarm to waist and underarm to bottom of the shirt so you can add some shape to the sides, to make it fitted. Make sure to add seam allowances to all your measurements- about half an inch on each edge should be enough.

In the picture, I showed three basic shapes you need to cut out- the main body shape, the collar and the waistband. the pieces for the body are all about the same, but on the front two you might want to make a gentle curve ont he front seam, so it gets a little wider at the boobs and a little slimmer at the waist- again, just so it fits better. If you make it too much wider at the boobs, it'll just get puffy and weird, though, so don't overdo it- if you have massive boobs, you'll need to put in darts which I'm no good at explaining. If you need help, though, I'm sure some friendly craftster will be happy to help you.
So anyway, to make the peices: The collar and waistband are just long rectangles. The body peice is a bit more complicated. The bottom edge should be 1/4 of the total bottom opening measurement, plus seam allowances. fromt he underarm to the bottom of the shirt should be an inward curve, and of course the neck opening should be curved.

OK, so now that I've made it seem way too complicated, let's get to the good stuff: you should draw these shapes onto either light newsprint, tracing or tissue paper, or onto cheap muslin. I'd suggest using muslin and then sewing the main pieces together to check the fit. You'll just have to take them apart again to make it with your real fabric, though, so I don't know if you want to bother with that.
Either way, pin the pattern pieces to your good fabric and cut it out. Now you can get to the fun bit: actually sewing the thing.

First off, decide where you want the zipper. Whichever seam you decide to put it in, leave that one open and sew the other three body seams. If you decide to try darts, put them in before sewing the pieces together. Next, pin and sew on the collar and waistband, and hem any raw edges. Now you can put in your zipper, and you're done! Wear it with pride!

Whew! That was long. I hope it made at least some sense. If you have any questions, just ask.