Beat the demo level if you haven't already.
Go to the Main Menu.
Select Endless Mode - Very Easy.
Grab all the pickups when you can, shoot all the enemies (don't run them over with the bike - it's hard to steer into enemies and you'll get shot trying), find cover as needed, and for the love of God don't drop any landmines (they kill you easily). After a few tries at this, you should get to wave 5 and survive it without dying.
Getting to wave 10 is another matter entirely, with gas mask horses and attack helicopters and hordes of hostiles trying (and succeeding) to kill you first.