Well, first I'd recommend you get a ton of reference pics, you're going to need those.
Then I guess you'd need the armor. There are a few sites that have pepakura files of TitanFall armor. Like this one: http://www.405th.com/showthread.php?t=41091
Pepakura... or you could make them out of foam. Whichever you're more comfortable with.
For the clothes, you might want to look for existing clothing that resembles what the pilots wear. Maybe look in thrift stores? You never know what you may find.
I hope this helps, good luck!! I bet it'll turn out awesome!