A few tips to get you started....
Colour - for me this is the most important ingredient in my Quilt. I have chosen my favourite colour combination pinks, greens, blues & yellows with a hint of blue and white - I had also toyed with white-on-white - sometimes I am such a girl! I also have favourites (fabric & garments) which I know will definitely be used. Start with a palette you like - you can always add to it later.
Fabric - keep like with like. If using heavier fabrics like denim you may want to think twice about stitching a light cotton to it. I have chosen mostly med.weight cottons however the other Quilt I did similar to this was mostly denim jeans.
Condition - if you start with quality you'll end with quality. Make sure the garments you are using are clean and that any marks or holes are taken into account - you can cut around these or stitch over them if you know where they are.
Content - the more you have the more you can play with!
Consider mixing your fibres ie. T-shirts can be stablised with interfacing or free-machine embroidered into and like Knitwear could be used as a feature appliqued for interest.
Come on, jump in - it's going to be alot more fun with you!