Its best to repot maples in early spring when you first see buds. No need to repot unless the soil is not draining or the roots are pot bound. otherwise, repot every 2-3 years. For the 18 incher, if you cut the apex, it will probably grow shoots lower, but there are no guarantees. when in doubt, let it grow, you can always cut later. Maples are good for beginners, they grow fairly fast. keep it outside year round, dont overwater (only water when the soil is almost dry), give it plenty of shade in the hot months and shield it from deep frost and wind in winter. good luck.