Wire the branches as soon as you can, don't let it dry out, let the shoots extend for a bit this year to increase vigor. Pick the flowers to encourage shoot growth. Watch out for cracks in the branches after wiring. In my experience, too much work too soon slows the plant down, that is why I let the shoots go a bit when they are young.
Marc, This is a small barberry called yellow sunshine (if I remember right) I bought it as a nursery plant about 5 years ago, cut the top off, cut the roots off and put it in a small pot, trained a side branch up for the new apex and the tree never missed a beat. It flowers in the spring and puts on a few berries. Nice easy small tree.