This first mission was just like Halo 1's 2nd mission where you had to find the marines that crash landed throughout the map, but in halo reach you are finding spartans. It's a pretty big map, and the worthog was very useful for getting places. The worthog can carry as many as 5 people (the driver, the front passenger, the gunner, and two seats below the gunner that face left and right, not forward)
The second mission was pretty action packed. You start out getting dropped off, and the action immediately begins. You are with squadmates. Throughout this mission you have to cover your spartan friend (whos a girl) and give her coverage so that she can hack into the computer. There are 3 big guns on the map that she has to hack into. Hacking into them gains control of the gun, and I guess it gives them control of the air (you see banshees and hornets flying throughout the entire level)
The third stage is very similar to Truth and Reconciliation (the 3rd stage on Halo 1). You and another spartan have snipers. You have to get into the enemy base from the cliffs (you start out on the cliffs). While you go in the base, your spartan friend covers you from the cliffs above. At the end of this level, you save a bunch of scientists or marines (not really sure who they are actually). They are humans who refused to leave their site even though they ordered everyone to get out. At the end of the level you save them and get picked up by a pelican.