Click on "My Wikitree" after you sign in. At the top of the page there will be a Search Field that has your last name but no first name. Clear your last name then put in the name you want to search then click on the magnifying glass.
That will take you to all those of that name on Wikitree. If it also gives too many similar surnames you can go down to the bottom of the page and ask for exact search specification such as dates, surname variants, etc.
If you still have problems, ask again and we'll do more to help you.