Ok, the first step is the click on the folder beside the textbox on Output. Click on the format you want to save it as.
The second step is to upload your video. Click on the folder beside the textbox on Input. Look for the video. One you found it. click on the picture, then click on open.
The last step is to encode. Click on encode and then a funny man below will start moving. When its done, under "output", a button should come and say "preview". click on that to view the video. The Video will be save were you uploaded the old video from.
I hope this helps. If not, just ask me. And I will help out ^^;;