You can negotiate the parameters of your submission for a particular scene (say a spanking), or for an hour or an evening, or for life.
You answered this question in your previous question--you get to fulfill your desires. Whether you are on Top or on bottom or switch between the roles, you are enjoying the erotic possibilities.
The line is where you negotiate it to be. Do you only want to do D/s in the bedroom? Do you want the littlest things in your life controlled or to control their lives? You also have to balance with real life--most of us still have to work for a living, deal with family issues, and take care of ourselves in addition to whoever we partner with.
There are as many ways to Dominate and submit as there are people in the world. It just depends on what floats your boat.
You build that trust over time. It doesn't just happen and it does take work. Don't think that you can just turn a switch and you will be a slave or an owner. As you get to know a play partner, each time you can open yourself a little more.
Hope that helps.