In any profession, it is important to be aware of the professional organizations that are related to your field. Although you might have to work for several years to get a chance to join the Costume Designers Guild, many of these organizations have student or junior memberships available. Be aware that not every group that calls itself a guild is a professional organization.
There are far more professional guilds and societies related to costuming than I am currently aware of, but here are at least a few of them. I will add more as I find them or people introduce me to them. These links lead to external websites for costuming related organizations.
Being listed here is not meant to represent any kind of endorsement on their part, or mine