On the page that Doug gave you a link to, 'Project:DNA', look at the section that says:
DNA Tests, Have you taken one? Please add yourself at Special:DNATests.
If you click on the Special:DNATests link when you're logged in to WikiTree, it will take you to the place on your profile where you can enter which DNA tests you've taken.
For the potential 2nd - 4th cousins Family Finder suggests, yes, you need to contact them. Tell them a brief version of your surnames & locations and ask if they recognize any of them. If you change your privacy level on WikiTree (for how to do this, see the page Doug sent you to) then you will be able share your Tree with others who are interested.