After having taken lessons from a number of instructors, and given lessons at a number of studios, I now know what I wish I had known when I started. At least I can pass on a few jewels of wisdom to you.
Do I need a partner? The first thing you should know is that you do not need a dance partner to learn how to dance. Dance studios typically have group classes and many of the students are there without a partner. Also, during group classes students are usually rotated every couple of minutes so you are dancing with different partners. This is good because you get to experience dancing with different people, which means you will learn to be better at leading (or, for the ladies, at following.) If you are taking private lessons, your instructor will be your partner.
How do I get started? The best way to get started is to take some group classes. For this you can either go to one of the local dance studios, or you can take an evening class at a community college. If you go to a dance studio, be sure to try out as many dances as you can. If you take the college class, you will get to experience several different dances. Once you have a good idea of what the different dances are and what they entail, you will also have an idea of what you want to do next with your dancing.
Where do I go? Three excellent studios in the Sacramento area are The Ballroom of Sacramento, Capital Dance Center, and VIP Dance Academy. The Ballroom and CDC have dance parties on Saturday nights so you can get in a lot of practice.
There are other studios around Sacramento as well, but I can only talk about those with which I am personally familiar. I have provided some links above to several dance studios, many of them in the Sacramento area, as well as other dance resources. I haven't been to the Starlight Dance Studio in San Diego, but I ran across some clips from instructional videos on Youtube that were very good, and that led me to their site. Next time I am in San Diego, I will definitely have to stop by.
Private Lessons? Do not underestimate the value of private lessons. If you really want to get very far in your dancing, you will need to take private lessons. That is where you will get your individualized attention; you can learn at YOUR speed, and really get your technique dialed in.
What about rates? That can vary not only form studio to studio, but from instructor to instructor. Group classes typically run from $8-$10 per person. Private lessons in most studios will run from $45-$80 for a 55-minute lesson. The better and more in-demand the instructor, the higher the rate. If you DO have a partner, rates for two are the same for one for private lessons. The single notable exception to these rates is Arthur Murray Dance Studio. Group classes and dance parties are free, but really are only for students taking private lessons, or those checking out the studio. Private lessons there vary only slightly, from $110-$120 per 40-minute lesson, depending on how many lessons you contract to purchase.
For experienced dancers: A great place to learn new moves is on Youtube. You can always get an instructor to help you fine-tune it later.