You need to be registered so when you're ready to submit your armor that's were you'll submit it. You'll have to submit your armor to a garrison who with then submit it to the 501st.
http://www.501st.com/databank/Membership:JoinUs
I've got my costume, now what?
Once you have your costume finished, you need to prepare your photos and submit your application.
Preparing your photographs
Your local Garrison Membership Liaison (GML) will need to verify your qualifications and will will ask for pictures of you in your costume for validation. See the 501st Legion unit listings to identify your local GML.
The following tips will help make sure that you get the right photos to the GML on your first try:
Only submit pictures of yourself alone against a plain background (no convention photos, please).
We will need photos with your helmet/mask on and off (if applicable).
You must include full-length views of your costumefrom the front, back and both sides.
Be sure to take your photos with adequate lighting and at a high-resolution to help us inspect the details.
Do not wear any non-canon accessories or add ons, including jewellery, glasses, earrings, (unit patches, extra weapons, pauldrons when not part of the canon costume etc...)
If you include any weapons such as a blaster or lightsaber, please ensure that they conform to the weapons policy.
Submitting your application
Once your GML confirms that your suit is complete and valid for membership, fill out the online application.
When your application is approved, you will receive confirmation via e-mail. Your Legion ID will be assigned and your record marked as "ACTIVE" in the 501st Legion member database. Please note that the 501st is an all-volunteer organization and the application may take several weeks to review.
If for some reason you are not validated for membership, your local GML will work with you and provide recommendations and resources that can assist you in modifying your costume to meet Legion requirements.