If your just starting to develop for Quake and QuakeDC for the first time you'll need to decide what area to work in first.

QC is the most common form of development for Quake. It is the programming language used by quake to control all the items, monsters, doors, the player's view and movement, and much more. Its is by far the most flexible thing in Quake. It doesn't take any real skill to do some basic modifications which can dramatically change the gameplay. Coding newbies can get tutorials for adding and changing various things. Experienced C and C++ coders will easily recognize the structure of QC and may even laugh at its simplicity.

Mapping is probably the most visually appealing way to mod for Quake. There are some maps so beautiful that you may well forget this is a game meant for First Person kill or be killed combat. Of course if your just starting out you wont be building the most beautiful (or even playable) maps. With some time and effort mapping can become an easy and enjoyable hobby. With a little extra effort the same Quake map could be adapted, compiled, and run on Quake2, Quake3, Halflife, Sin, HexenII and most any other current Id software engine based game. You can consider Quake1 map authoring as a stepping stone into the world of First Person Shooter mapping for that reason. Another thing to note... some of the better Quake map authors have moved on to a carrier as a professional level designer.

Possibly the most common form of 'development' for Quake is that of a Texture Artist. Textures are used by every map and model in most every 3d game you've ever played. Quake is no exception. Sure its not really development... but it is important. At this time details on how to become a Texture Artist will not be given here. Basically look around for tutorials (doesn't matter for what game 99% of the info should be interchangeable) and mess around with photoshop. Just remember Quake has a limited palette.

Possibly the most difficult (and expensive) thing to get into is model editing. The few Quake model editors available do not make building or editing models easy. And are not the best way to model. The best program for making models for any game is probably 3D Studio Max (in combination with Character Studio) which is a program that can cost 1000's of dollars. There also isn't much info on modeling for Quake available.
There is a shortage of modelers for Quake though...
If you can find the info and are willing to do the work there are probably tons of moding groups and individuals who would love your contributions.