Bike Bags and Backpacks
The simplest way to carry anything on your bike is with a backpack or messenger (satchel) bag. For smaller items and laptops the messenger bag is ideal as they are shaped, padded, can be fastened for security and waterproofing, and sit comfortably and securely against the body with the across the shoulder strap. For slightly bulkier items such as a change of clothes, a backpack is ideal and cycling specific models are packed with features to make them more comfortable while riding as well as securing items to them like helmets or waterbottles.
If you need a larger kit bag for travelling then there are a range of manufacturers making models with useful cycling related features such as padded helmet storage, shoe compartments, ventilated areas (handy for wet gear)as well as the usual cavernous areas that still seem to fill up too easily!