It all depends on the fabric, when I'm looking at it I usually envision projects for it.... If I think it would make great borders I buy 3 yards, if it is a basic that could be backgrounds, or used in many different quilts I buy 3-5 yards ( usually depending on how much I love it & price) if I love it but really can not think of what I would do with it I might just buy a fat quarter, or half yard, just to have it - as time goes by I always find a use for it- if I love a fabric but feel its use is limited ( great for applique or fussy cutting) I might buy 1-2 yards. Being new maybe you should browse patterns you love, see how much of different fabrics are needed and keep those amounts in mind while browsing. When you have an idea how much it takes for borders, blocks, bindings it is easier to determine how much to pick up ( plus a little extra for mistakes and stash)