Lots of our Members have done ODST costumes.
You can use the search function on the Forums to find threads created by other members to document the build of their ODST Costumes or you can look through te Halo Costumes and Props Section:
Discuss how to make, what steps to take and where to go from here. Post only after you have read up on the topic thoroughly.
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Like Chris mentioned, a good place to start before even talking about an ODST costume specifically is to read through the Tutorial Section and learn about all the various ways you can make armor:
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Also, take your time. I see you are only 13 years old, and appear to be very excited. That's great. We also love Halo costumes and are excited about them, but do realize that most of our Members here are adults. So, firstly, make sure your parents are okay with you being here and interacting on our site, and then take that into account as you navigate the site and learn about how to make your costume that many of the folks you are speaking with are adults, many with children of their own, and try to pace yourself. Really take the time to read and research and learn about our community and how to make armor and all the safety precautions you need to observe when doing so.