If you're going to start making add-ons then I recommend you not release any of the first products unless you are ABSOLUTELY CERTAIN they don't have any errors in them. If you don't even know how to check for errors then just don't release anything at all. Furthermore, I recommend you not release anything unless it meets some simple criteria I'll list shortly.
Anyways as for add-on making in general it will depend on what content you want to work on, which come in some general flavours
- Items/vehicles/weapons (includes emitters and lights and other simple modifications)
- Maps
- Decals
- More elaborate functions and scripts (GUI systems, Events, complex utilities like the duplicator, etc.)
The 3 first parts can be handled easily by anyone and everyone but you'll probably be taking a wack at the first item on the list. All you'll need in that case is a Model with the correct nodes/joints and a script to go with it. Map making requires some particular programs, or if you know how to use the map editor you can change current maps (not recommended). Decal making is easy, however making
good looking decals is hard. The last part requires some in-depth knowledge of Scripting, more particularly in the C++ field. I don't think you're ready for that yet.
*** Most people make a nice model, sure, but they neglect the script and just insert a pre-made script to go with the model (gun script, minigun script, rocket launcher script, etc.). NO ONE WILL DOWNLOAD YOUR ADD-ON IF IT'S ONLY A MODEL EDIT, EVEN IF THE MODEL IS FANTASTIC FOR WHATEVER REASON.
In all honesty, the current add-ons on blockland could look and handle allot better in almost every aspect, not just visually. Weapon and Vehicle scripts are probably the most overlooked aspects of add-ons, which is a shame because some weapon models are very pretty but end up firing gun bullets with rocket launcher explosions. Scripts are currently a copy-paste job in most cases. Please be original. If you want to make the absolute best add-ons then you'll have to scrutinize Everything, especially the script.
Things you'll need to make a good weapon:
- Originality ; and for that you'll need some good ideas. Go check out Suggestions and Requests if you can't think of anything.
- Custom weapon model and Custom projectile model ; if it's required
- Custom script ; with it's own unique Emitters/Explosion effects, projectile effects, imageStates, etc.)
- Needs to be something like a toy ; as in Blocko. Not a WW2 m80 grahand 52K4 or whatever generic gun. There are more then enough of those already
- *** Needs to work ***
Vehicles are the same deal, however they are much more complex so we can expect you'll be fluent in this business by the time you become able to make vehicles at all.
There are several guides to modelling and general add-on making already on these forums. Use the

button at the top of the page to search for specific items. Script related problems go in Coding Help. Model related is usually in General Modification Help. Modification Discussion is used to discuss projects and whatnot, check it out if you want to see what others are up to.
Finally, diligence pays off in the end. A single awesome add-on beats 10 other add-ons that you just pumped out on a "whim", so please invest time and effort into making them. You'll be up and making quality add-ons in no time, but in the meantime be patient and experiment only.