Ok, I'll try and not confuse you further, lol.
Lessee...well for starters there's Orbital Drop Shock Troopers (ODST
for short) and they are basically the Special Ops of the UNSC (Human military). I'm assuming that's what you were talking about when you said you were going to make a trooper.
ODST's take on the more dangerous jobs that the regular Marine's are not qualified to deal with as I understand. They do infiltration (armed attacks to take over a building, city ect.), ex-filtration (search and rescue stuff), demolition (blowing stuff up) and they do high altitude drops into war zones.
As for the story, the ship hasn't crashed, we've just started the beginning mission and have used SOEIV's (Single Occupant Exoatmospheric Insertion Vehicles or simply "Drop Pods
". ((Exoatmospheric = A fancy word for extremely high altitude))
Here's a picture of what it looks like: http://www.creativeuncut.com/gallery-11/art/hodst-soeiv-pod.jpg
As you can see not exactly a space ship, think of it like a life pod big enough for 1 person.The Marine's
are simple, they do as they're told, make raunchy jokes and shoot a lot basically. Atleast, that's what they've done in the series *shrugs*. I'm sure actual Marines IRL do much much more but, that's what I know.Spartan's
are genetically enhanced super soldiers and they are black ops. No one is allowed to confirm their actions on record or in written record. They never existed type stuff.
I hope that clears some of it up for you.