If you don't want to make your own signature, there's some threads where people are making ones for free. Here are 2 examples:
Am1og
spodermon
Make sure you use the full template given in the thread if you want one made from one of these ^^
If you want to make your own signature, I can't help you.
Once you have the image upload it to
Imgur
To use the signature, go to Edit My Profile --> Profile Information --> Scroll down to the bottom and there's a signature box.
Then type:
If that doesn't work, you need to put '.png' at the end of the link.
I hope this helps!