Return to the Shadows is developped in a really open way. You are free to contribute, as long as you've got the skills and your work fits into the game. Currently there are three main sections in which contributions are needed: maps, graphics and sound.

If you have any contributions, mail them to prefect@rtts.org, and I will add them as soon as possible. I will of course add your name to the credits.

Mapping is the process of building one of the levels the player is going to fly in. Mapping is something that anyone can do. You don't need any special tools at all, and it's really easy to get started.

A step-by-step guide to the editor can be found here.

Right now, maps for the levels 4 to 10 are needed the most. Starting with level 11 or so, previous maps can reappear, with more and tougher enemies.

There are many different graphics in RTTS, but the most important ones certainly are the tiles (those are the bitmaps which the ground is composed of) and enemy graphics. You will probably use PaintShop, Photoshop or similar when creating the actual graphics.

In many cases, there are "glue files", consisting of plain text which tell the game code how to use the graphics in-game. Those glue files aren't well documented, so I suggest you bug me about that until I get the documentation done.

I normally accept all contributed enemy types and tilesets. If you intend to do something more out of the order or even radical, it would be wise to explain your plans on the forums before you start lest you end up disappointed because your idea was rejected.

Recommended reading:
Creating new tilesets

Currently, RTTS has no sound at all, simply because nobody with the required talent has shown up yet. The one who contributes sound effects for RTTS is going to be the hero of all players out there ;)

Background music is not a high priority right now. It will be supported (as MP3 and OggVorbis) eventually, but since you can just run an MP3 player in the background while playing RTTS, it's really not that important.

What Return to the Shadows does need are sound effects: the sound of firing guns/rockets, sound effects for rocket impacts, and so on.

As you will probably have noticed, the source code for Return to the Shadows is open. If you actually want to contribute code (apart from bugfixes etc...) you should open a thread on the forum or contact me first. I can be rather anal when it comes to code design and coding style.

There is more information on the Sourcecode page.