The best place to start is Cobl/Modding This page lists all of the major packages and includes links to subpages which cover more complicated topics.
If this isn't enough, ask on the forums for additional help. (Do not post help requests on the wiki. Use the forums instead.)
Where do I find Testing Cells?
Generally, it's best to make your own testing cells. However, if you would like to visit Cobl's testing cell, coc to cobTestCell. Do not add anything to this room!
Should I Include Cobl with my Mod?
NO! Do not do this. Instead, you'll need to direct users to download Cobl and install it themselves. Keep in mind that many (if not most) users will already have Cobl installed.
The reason for not including Cobl is so that Cobl can be indepedently updated of any mod that depends on it. And since Cobl is designed to be backward compatible with mods built on top of earlier versions of itself, there won't be any problem with incompatibilities.
Note that the independent updating of Cobl, means that behavior of Cobl components used in your mod will likely improve over time. E.g. Mods created in early 2006 to use Cobl's alchemical sorter have all benefitted from recent improvments in the sorter without requiring any update on their part.
How to Indicate Cobl Dependency?
You should clearly indicate in the readme for your mod that the mod requires Cobl (and what version of Cobl is required). In the installation instructions, you might want to link to the Cobl readme. A good point to link to is the Introduction -- it's short and the install instructions are immediately beneath it.
Also, in the description field for your mod, should include two lines at the top: a Version line followed by a Requires line. E.g. for Wrye Shivering Death:
Requires: Cobl 1.18
This changes the way you die after completing the Shivering Isles Main Quest.
The Version line should be the internal version number of your mod. The Requires line should indicate the minimal version of Cobl required for your mod.
When specifying the version of Cobl required, it's safe to specify the version of Cobl that you built on top of. However, if you know that all Cobl features that you're using were available in older versions of Cobl, you can specify that version instead. (E.g. if you're only using the alchemical sorter, you can specify Cobl version 1.00.)
Including version and requires info in the description is useful since it can be used by tools like Wrye Bash in mod list postings and to help debug load lists.