How can I create a logo for my wiki?
Do you mean a wordmark? You can create it in any image program.
A wordmark should be a 250 pixel wide by 65 pixel tall (250x65) PNG (.png aka Portable Network Graphic).
Here's some info on how to upload the wordmark to your wiki once you've made it: How to upload a logo to your wiki.
I use GIMP to create my wordmark images; it's a free program. Unfortunately, learning to effectively use such a program can't be explained in an extremely concise manner; you might try practicing use of GIMP, Photoshop, or Paint.NET for a while until you get the hang of it, or you could ask the community to make you a wordmark.
Thanks Thisismyrofl :)