Pick out your fabrics first. The fabric you choose for your design has a big impact on the look, fit, feel, and construction of the garment.
A denim dress is a very different piece than a satin special occasion dress. The designs could be overall very similar, but the different fabrics lead them in completely different directions. The construction will be different. You’d probably choose different types of thread and topstitching, too.
This is why choosing a fabric is one of the first things you do when creating a new product. Fabric sourcing can happen before or in tandem with design. Certainly before you start any technical design or patternmaking, you need to know what fabric you will be using.