As the old saying goes, practice makes perfect. Before you open up a real "Shop" I would recommend doing signatures for free to people to improve your skill. Once you personally feel confident enough to open a shop, which should come off the complaints/reviews of the people receiving the signature, then you should.
With that in mind, the render tends to make a signature. You have a lot of extra white that wasn't cut-out of the original image. For example, in your first image, you have a small white dot above the "Y" and there is a completely uncut section above her left hand between her hair. In the latter, you have an odd cut-off of your render below the "as" in "Xmas!" and apparent white dots below the "n." Additionally, the edges look very forcibly cut--which is never a good thing--so I recommend fading out the outside so it "connects" better with the background.
In terms of borders, I have always found that a 2-3 px around black border to be the best, borders that are too bright tend to distract the person(s) viewing the signature from the main content. (My recommendation is make them smaller/darker color.)
In terms of font, a lot of people tend to go overboard with this. Sometimes the best font is the most simple, sometimes not. This depends on a multitude of things involving the signature. At very least play with it some in all of your signatures and see what happens.
Background is also another highly important thing, of course. It's always hard to find the balance between a background that is taking up too much attention or not enough. In most cases you want a background that blends or connects to the foreground (which tends to be a character of sorts), but this also varies case to case. For you personally, try to add a bit more too it--there isn't much there.
I personally have never used tutorials myself, but I know a lot of people like to. I suggest you look some up on Google or whichever search engine you use.
Finally, what program are you using currently?