Well, your question is slightly better than most new people who come to the boards.. But not quite good enough i'm afraid. The best think i could tell you is to familiarize yourself with the commands of darkbasic/pro(not sure which your using), run the example programs that came with it and try and understand the code that went with the program.
Chances are if you can recreate those programs, you'll get much farther at creating the games you'd like to make. On another note, i wouldnt try too hard to make a halo clone, atleast not graphics wise... Dont think darkbasic will be able to handle it, but something similar and less graphics intensive could be done.
Good luck, and welcome to the forums Angie
WindowsXP Home(Service pack 2), Athlon XP 2400+(2.1Ghz), 1GIG Ram, Ati Radeon 9800Pro 128MB.