Garter stitch is when all rows are in knit stitch, rather than alternated with rows of purl. The needle with the caston stitches should be in your left hand and the yarn at the back of the work.
1 Insert the right needle through the first stitch on the left needle, from front to back (so the two needles are crossed).
2 Take the end of the yarn and wrap it clockwise over the top of the right needle (behind the left needle). Keeping the end of the wrapped yarn in your right hand, pull the right needle towards you, bringing the needle tip in front of and above the left needle: this creates a loop on the right needle.
3 Release the old stitch off the left needle.
4 Repeat steps 1 to 3, until all stitches are on the right needle.
5 Next, swap the needles around and repeat steps above.