Signature is a picture, you post it via your account management / signature, just like posting pictures in thread. For that you need picture's address (what pzykozomatic said). As for picture itself - you can use already existing pic from the net, you can use specially prepared ones, like Inara provides. You can also make your own but for that you need tools - namely graphic design software. Simple MS Paint will do but don't expect much from it. Other software is: Adobe Photoshop/Illustrator, CorelDRAW, Affinity (all paid products) or free opensource like Inkscape or GIMP.
But those require bit of knowledge, you need to know signature dimensions (most cases given in points, not common mm) and program itself. With basic skills you can arrange a rectangular shape, fill with color, add cropped photo, add text and whatever you like. Save your project, export to proper file type (jpg or png), drop into hosting site, get the picture link code.
Then you can put that link code into your signature field under account management. If done right forum site should read the code and display the picture as your signature. Currently I'm in the middle of figuring out what I have messed up since my updated sig is not showing up at all.
