Buy fabric first. Then decide what to make. Why? Because fabric is harder to find.
But how do I know how much to buy if I don’t know what I want? As a rule of thumb I buy fabric in lots of 1m, 1.5m, 2m or 2.5m. If I really like it and it’s cheap I’ll get 3 or 4 metres.
Roughly I can get a little top or a mini-skirt out of 1m, a pair of shorts out of 1.5m and shirt from 2m. If I think I want a dress I’ll go for 2m.