Welcome to the forums. You are asking a couple different questions, so let's tackle them one at a time:
1. How do I make scripts? Well, for starters, there are a few tutorials floating around out here on scripting, not to mention the ones in the Community Guide. I'd suggest reading those. As for what program to use to write them, I'd recommend FPI Editpad.
2. How do I add pre-made scripts to the editor/map/game? First you need to place an entity that you want to use a script on. Then you need to right click that entity and edit the properties. Then, you have to select Main, and then you have to.... You know what? The Community Guide has a great section on adding scripts to an entity, so there's really no point in me repeating it here. I'd suggest reading that.
3. Can I have my own scripts available to me? Yes, you can. You just need to remember to save them in the Scriptbank folder on your PC.
Again, I'd suggest reading the community guide. It has a lot of the answers you are looking for.
Priest of the Church of Joe Wood