Because SCA events are intended to recreate similar occasions in the past, everyone attending an event is expected to wear medieval clothing, called "garb" in the SCA. This helps to create the atmosphere and is part of the fun of events.
At the beginning you will probably be able to borrow garb to see if the SCA is for you. However, eventually you will want your own. While you can sometimes purchase garb at events from merchants, most people in the SCA end up making their own garb.
For Canton meetings in Harrowgate Heath, garb is encouraged but modern clothing is fine.
Here are some helpful sites for making that first set of garb:
How to Make a Quick-and-Dirty T-Tunic
Excellent how-to article on making that first piece of garb (with clear diagrams)!
The Grey Company
Another quick-and-dirty T-Tunic page, also with easy-to-follow instructions and diagrams.
Duchess Leah Kasmira of Natterhelm's Basic T-Tunic Page
Easy-to-follow instructions with drawings.
'T-tunic' - the period way, By: Lady Muireann ingen Eoghain ua Maoil Mheaghna
This is an authentic type of T-tunic pattern also known as a 'Bog' Tunic because they have been reconstructed from clothing remains found in peat bogs. This one was found in Bocksten, Sweden.
Some Clothing of the Middle Ages - Tunics/Cotes
LOTS of medieval tunic patterns!
Clothing and Costume Links
Check out the "Costuming for Beginners" section - lots of great links for easy-to-make garb!
Articles and Essays of Interest to Costumers
Lots of good articles on Hats & Hair, Clothing, Accessories and Essays, Trip Reports, and Miscellanea.