Question 1:
Go to File - Save As - Choose a filetype of what you want.
Question 2:
It's advanced graphics, it supports layers and complex images, tablets, icons and cursers with a plugin, scanning, editing, all of the above.
Question 3:
Go to the tutorial forum and read the Tutorial on "Lets make some firy Text with Paint Net"
Take your text, open your jpg image, make a new layer in the layer window, and paste your text in, or do the firy text in the new layer before hand.
If you need help understanding layers, or the tools, in Paint .NET, go to Help - Help topics and read through them.
Pm me with any other simple questions if need be.