For models, get Milkshape3D
There are tutorials on how to use Milkshape to make blockland weapons lying around all over the forum, and they are pretty useful.
For scripting, you can learn the basics mostly by experimenting.
There is a tutorial on how to make a copy and replace the name of an already existent weapon, such as the gun, and then you can play around with the Datablocks to try and figure out how things work. Guns are mostly particle effects and once you grasp the basics you can make pretty explosions and neat stuff. (WARNING WARNING WARNING Do NOT overwrite ANY scripts, make a copy, paste the copy outside of your add-ons folder, RENAME IT, then modify it)
There's also a couple of syntax rules in scripting, a good example is one you will see often being the use of ; at the end of almost every line in a datablock. If you change a line, or create a new line, make sure you follow the syntax rules or you will have problems.
Afterwards there's the ItemData and ShapeBaseItemData, where all your datablocks come together and the magic begins. You need to be ultra careful here, because this is where you will most likely mess something up.
The two danger lines are the ones that have .dts files (your weapon shapes). If, as an example, you made a 'replace all' RocketLauncher with BallLauncher, it will also replace the RocketLauncher.dts with BallLauncher.dts. The problem here is that if there is no shape called BallLauncher.dts in your shapes folder, so leaving it as is will cause the weapon to simply not work. Of course, you can change this back to RocketLauncher.dts to get the rocket launcher model for your weapon, or some other name for another weapon shape in your Shapes folder.
Then there's WeaponImage, and States, which can be regarded as a chain of actions. There are no tutorials for this as of yet, that I've seen anyways.
