WikiTree is one tree for all - not individual trees like many other sites. I suggest that you begin with adding your parents, grandparents, etc. and eventually you will connect "your twig or branch" to someone already here and then you and your ancestors will be a part of the whole tree.
To answer your question, if you have information on profiles already on WikiTree, if the profiles are Open, you are welcome to add the information you have - be sure to have reliable sources to back up that information. If the profile is not open, then you can contact the profile manager and explain what you have and the sources. We strive for collaboration on shared profiles.
One caution - even if the profile is open and and there is a profile manager, if you plan to make any major changes rather than just adding helpful information, then I suggest that you contact the profile manager of the major changes you propose on that person.
Welcome to WikiTree - we look forward to your added branch.