Background story

[saphire krakens]

What sort of things should i put in a background story for my marines chapter?

[snowblizz]

I guess the silly answer is anything you want?

What are you looking to do with the background story.

But I guess the basics is where they came from, some defining moments that made them what they are, some random battle honours and main antagonists, one or two famous character from its history, some exposition on the characters you include in the army now. And some general points on what type of chapter they are and what motivates their intervention in the world of 40k.

[IBBoard]

As Snowblizz said, your background contains whatever you want for your chapter. The main points should all explain why they play how they play, what their ancestry is (which legion they descended from) and what their motivation is. Defining moments, famous characters, any enmity with other chapters or races, preferred weapons and fighting styles are just some of the ways that you can bring that out.

[saphire krakens]

Some things Ive got is special characters, some differences between the other chapters like the emergency teleportation system that sends the marine to the battle barge or strike cruiser when they die, about to die or manually activated, some philosophy, general tactics, and traditions or flight paths for the fleets
anything i should add?

[IBBoard]

That sounds like it covers most things, and is probably deeper than a lot of people go. You might want to cover iconography (e.g. squad badges and chapter markings), but you might have that already. At the moment then that's all I can think of.