Is your base a square or rectangle? If so, you probably have stitches on the needle that cover just one of the four sides of your base. If you are using straight needles, you now have to change to a long circular needle. Once your initial stiches are on the circular needle, you will need to go around the rest of the three sides and pick up stitches. Typically when you work the first side extra side, pick up one stitch for each row. Then on the second side, pick up one stitch for each stitch. On the third and final side, pick up one stitch for each row.
I've found it helpful to use a crochet hook to pull the working yarn through the work and then to slip that loop over my needle when picking up stitches. Especially when working around a square/rectangle. Things will get really jammed together on the corners, but after working a few rows in the round, that will ease itself out.
Once you've picked up stitches for the 3 sides, JOIN your work and begin working in the round. That is to say, once you've picked up your last stitch on the 3rd side, knit the first stitch that was on your needles before you began picking up stitches. Keep going in a circle, don't work back and forth anymore.
There is a demo video of how to pick up stitches here in the basic techniques ...and More section.
Hope this helps!!