Garter Stitch is as simple as knitting comes, and the good news is it doesn't get that much more complicated when you need to increase and decrease.
Because the fabric is already so textured, increases and decreases tend to hide in it, which is a very good thing most of the time.
I pretty much always use a knit in the front and back increase in Garter Stitch. The main complaint people sometimes have about this increase is that it shows a bump where you use it, but that's no matter in Garter Stitch, which is already bumpy.
The photo illustrates two increases -- kfb and m1 -- as well as two decreases -- ssk and k2tog. They are difficult to see, as I mentioned, because of the nature of Garter Stitch. That means you don't have to worry if a pattern doesn't specify what kind of increase or decrease to use; just use whatever you are most comfortable with.