There are a variety of ways to do this;
1. You can simply find signatures, the most common are "Userbars";
Userbars.com - The original userbar site with thousands of free userbars, forum signatures, photoshop tutorials, animator, userbar maker and more!
2. You can create your own on a photo editing program such as Photoshop
Photo Editing, Photo Sharing | Photoshop.com
Scroll down to "Try and buy" then go to Photoshop CS5 - Free Trial.
To understand the basics on how to use this program you may want to check this section out;
Tutorials - MPGH - MultiPlayer Game Hacking
3. To request a signature made on your behalf you may want to go here;
Help & Requests - MPGH - MultiPlayer Game Hacking
To put the signature up, simply save your file, go to an image upload site such as tinypic or imgur;
TinyPic - Free Image Hosting, Photo Sharing & Video Hosting
imgur: the simple image sharer
And copy and paste the [img]URL HERE[/img] code, Paste this into the 'Edit signature" Slot;