You mostly have two good options:

• Modding: Develop a mod independently and publish it on the forums.
• Core dev: Think or ask of something that is needed in the core, talk with the core team (preferably a lot, with multiple members - communication is more important than you think) and do it.

Upstream repositories and official Windows packages are handled by the core team, consisting of a bunch of people who are much trusted to keep Minetest progressing in good condition. The core team is best contacted on IRC at #minetest-dev @ Freenode.

The core team is formed of people who have made great contributions to Minetest.

Minetest is distributed as an engine, combined with a couple of games. Upstream repositories can be found at https://github.com/minetest/.

• The engine (core) is the base for everything. C++ is used for housekeeping and performance-critical stuff, Lua for extensible things.
• Games define game content: nodes, entities, textures, meshes, sounds and custom behavior implemented in Lua.

Games consist of mods that plug into the engine using the Modding API.