There are tons of ways to make bobbles. In general, to create a bobble you rapidly increase into a single stitch, work these stitches for two or more rows and then decrease them back to one stitch. This tutorial demonstrates how to make a 5-stitch Bobble in garter stitch.
With the right side facing you, knit into the stitch where you'd like your bobble, keeping the stitch on the left needle, yarn over and knit into the same stitch again, yarn over and knit into the stitch for a third time.
You now have 5 stitches where you initially had one.
Turn the work so the wrong side is facing you.
Knit the 5 bobble stitches.
Turn the work so the right side is facing you.
Knit the first two bobble stitches together. Place this stitch back onto the left needle.
Pass the remaining three stitches, one at a time, over the knit-together stitch and off the needle, as if to bind off.
Knit the remaining bobble stitch, returning it to the right needle.