Go to the "My WikiTree" tab, scroll down to "GEDCOMS," follow the directions. I suggest you make your first upload very small, maybe immediate family a few generations back. A very large file takes a very long time to do the things which need to be done after the upload. I also suggest before you upload, read/study the linked pages on the "GEDCOMS" page so you will know more about what uploading a GEDCOM to WikiTree entails.