A number of interesting wearing options can be created by having a slit in a scarf, and it doesn't compromise warmth. But putting in a slit isn't necessarily something that's taught to beginning knitters, so here's a quick tutorial. It assumes that you can knit, purl, cast on, bind off, and join new yarn, and that you have a basic grasp of things like gauge and ribbing.
First, decide how you want the slit to be oriented. Do you want it to be horizontal, between two rows, or vertical, between two stitches? A horizontal slit will go across the short width of the piece, while a vertical one will be parallel to the long sides.
A horizontal slit is pretty easy to create: when you get to the length you want before the slit, bind off stitches in the middle on one row, and cast on again on the following row. You should leave at least a few inches' stitches on each side-this will vary according to your gauge, of course, but for something in the 4 to 5 stitch-per-inch range about ten stitches would work. Your choice of bind-off and caston, though it's probably most harmonious to use the same BO and CO you're using for the piece as a whole. Keep in mind that the edges of the slit will curl if you're using a curling stitch pattern like stockinette, so you might want to add a few rows of ribbing, garter, seed or some other non-curling pattern before and after.
A vertical slit is slightly more complicated, but only slightly. When you get to the row where you want your slit to start, work across to the point where you want it (which doesn't have to be smack in the middle, but again you probably want to have at least an inch or two between the edge of the piece and the slit). Once you get there, turn your work as if you'd gotten all the way to the end of the row, and work back. Keep this up until you have a length of knitting that isn't the full width of your piece, but is as long as you want the slit to be. Stop when your yarn is at the edge you started from when you were beginning the slit, and break the yarn. Leaving the flap's stitches on a stitch holder or spare needle, go back to the stitches from the main body that you didn't work in your very first flap row. Join the yarn there and work just those stitches for as many rows as the first flap. Break the yarn again, put the first flap's stitches back on the left needle, and start working back and forth as if the slit weren't there at all. If you're doing something special with your selvedges, you'll probably want to do the same to the stitches along the sides of the slit, and again remember to watch out for curling.
That's really all there is to it.