Hard to say for sure without seeing all your code, but you should make sure to:
- disable lighting
- disable textures
- enable blending and set the right blend mode
Something like:
glDisable(GL_LIGHTING);
glDisable(GK_TEXTURE_2D);
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
If lighting is on and you don't have lights set up, well, everything will be very dark
If textures are on but you haven't bound one, I believe it'll pick up black for the texture color, and again, things will be black.
Something you can try to spot these more easily is set the clear color to something other than black - that way, you'll see black shapes against it if you mess up with lighting/textures.
It's also possible there's a problem with your view setup - i.e., you could be rendering just fine, but not looking at it.