Two Steps to Start
Create a new account
This is not necessary, per se, but if you have in mind to do more than just one or two sheets of adoptables, it might be a good idea to start a new account. This way you won't spam your watchers' inbox, if they're not interested in adoptables, and you can organize your gallery in a better way.
Be a member of Adoptables Groups
dA is a big community and the sheer amount of new deviations every day makes it hard for your art to be seen. Considering that you just made a new account for your adoptables, you have basically no watchers whatsoever so how can you get the exposure you need to sell your characters? The answer is join Adoptables Groups. By doing this you can do two things, the first is submit your adoptables to their gallery and advertise them, the second is observing the trends, what people like to draw and buy more.