As Doug indicated, it will help you a lot to read previous answers to this question. That will give the the most complete understanding.
The quick of it is, you would want to be logged onto your FamilySearch site, then find the option to download a gedcom. You might have to specify some specifics but it will be user friendly. The file might be sent right away or you might wait overnight while the file is generated, sent, and received through different servers and time zones. This gedcom gets saved to your computer. Edit it if you wish using desktop software. When you are ready, upload to wikitree. There are easy to follow instructions on this in the help section. Once uploaded, it will be checked for matches with the existing tree, then you are on your way!
I hope this helps,