What you wear is entirely up to you. There is only one requirement for participation in the SCA: wear something that is at least an attempt at pre-17th century clothing.
The historical periods of focus within the SCA cover a wide range of years and cultures, so there’s a wide variety of options available to you. Outfits are made or purchased. They can be made with period and modern materials. Some people choose to wear a simple belted tunic and baggy trousers. (The “peasant standard” for 2,000+ years) Others will go to great effort to achieve an authentic as possible appearance from head to toe.
Most of us are somewhere in the middle.
Some people wear styles from many different cultures and time periods. Others strictly adhere to the clothing that their “persona” would have worn. Still others will wear an Elizabethan gown to one event, a Norman tunic to the next, a Turkish belly-dancer garb to a hafla, etc. There’s really room for everyone’s interests – and the climate!
The terms “costume” and “garb” are often interchangeable. However, since we are trying to portray people of the past, its more likely that the term “garb” or “Period Clothing” will be used for the more accurate and authentic the garments.
For those of us who seek to understand people of the past by quite literally wearing their shoes, replication of historic clothing to a high degree of historic accuracy is essential. But this is by far the minority, and only enjoyed by those whose fun revolves around that eternal question: “Whatever Shall I Wear?”
All in all, we’re people to who dressing up in funny clothes and playing medieval games. It’s our idea of a good time. At whatever level you play, enjoy!