You could start looking for a studio by clicking into the Dance Directory on the left side of this web page. A bunch of studios are listed there.
You could also tell us what city you live in, and people reading this thread could make recommendations.
Other than that, you should look in the phone book and visit every possible studio. Since you've taken dance lessons before, you already have a sense of what makes a good teacher. Shop around, try out group classes at different studios, see which place has the right kind of teaching environment based on what you know to be good business and teaching practices. Talk to students, find out who they think the best teachers are, and try a couple of teachers before you settle on one for private instruction.
Is there a reason why you want to be in professional competitions as opposed to amateur ones? (I ask because I keep meeting people who have no idea that amateur ballroom competitions exist.) To get started as an amateur, you need to find a partner and a coach. The only way to do this is to get into the mix and start taking dance lessons and meeting people and looking for a partner. There are partner search web sties, including this one (click down under Classified Ads, to the left of this page).
The path to being a pro is longer, and the mimimum skill for entry level is very high -- there are no beginner levels in pro competition. As an amateur, you can dance in beginner competitions as soon as you want, but to be a pro will take getting yourself trained to the level of a pro (could take years), plus finding an appropriate pro partner (which as difficult if not more than finding an amateur partner) before you can even get started. I'm not saying it can't be done, but rather that it can be a long road so if you're doing this as a hobby, start out as an amateur.
