You can upload and retrieve files from the network using the Uplink CLI. Documentation for the CLI is located here and instructions on how to setup the Uplink CLI to upload your first file can be found here. Remember that you will need to first have created an account on a Satellite, created your project and added an Access Grant in order to be able to upload files to the network.
For background, an Uplink is an entry point into the Storj network. It connects to a specific Satellite and caches connections and resources, allowing users to create sessions.
If you are currently using Amazon S3, you can connect directly to Storj through an S3 gateway hosted along-side your application server. This means that switching to the decentralized cloud and reaping the cost, performance and privacy benefits is as easy as changing two lines in your config file! No new code needed! Documentation for the S3 Gateway is located here.