Hi there! Awful background music aside, this tutorial gives a pretty good explanation of how to make a GIF file specifically for use on Tumblr.
EDIT: Here's a very helpful one as well, if you want to use your own video clips (i.e. from a webcam).
You big tool is going to be the Timeline feature. You'll need a video clip which you'll then import into Photoshop
You'll be able to tweak some settings in the dialoge box that will come up. After this, the newly created frames will pop up in your Timeline frame:
Here you can set timing, delete frames, and make other edits (see the video tutorials for reference).
Then, you'll need to save your GIF for web:
You can add a watermark by adding a text layer (using the type tool, highlighted in red on the lefthand side of the screenshot below) above your frame layers, as shown. You can make this transparent by using the opacity slider in your layers panel.