I’m new to the SCA, what are good beginner basic garments I should have?

Shirt - In the middle ages, this long sleeved garment was worn against the skin
and was nearly always made of white/natural linen.  
Cotton can be used as an inexpensive but still comfortable substitution.

Tunic (knee length for men, ankle length for ladies) - This garment spans
many time periods and cultures and is a great starter piece for any persona.  
It is a versatile garment that allows for a wide range of sleeve,
color and decorative options.  It was typically made of wool, linen or silk
however cotton can be used as an inexpensive substitution.

Pants (for men) - As this garment has specific styles dependent upon
time and location, don’t make a huge investment
if you’re new and unsure of your persona. 
 Instead, cotton scrubs or sleeping pants can be a
quick fix for this challenging garment.