You don't even have to have had year of lessons. I've been in Pro/Am "Newcomer" events that were set up for people who have had less than 40 hours of instruction. And costumes weren't allowed. All you have to do to enter is have your teacher find one of these comps and sign you up (and of course you have to pay him/her for dancing with you and also to pay for the entry fees).

My ballroom club holds newcomer events for dancers dancing in their first amateur competition (although one of the partners can be more experienced). No costumes required, either. The only thing required is that you join the parent organization (USA Dance) to enter.