To become a host, you can either download the Sia-UI or the Sia Daemon at https://sia.tech/downloads.
For most users, running the Sia-UI is the easiest, because it provides a graphical interface. You can use the Hosting tab to set yourself up as a host.
If you are comfortable working in command line, you can just download the Sia Daemon and follow this guide https://blog.sia.tech/how-to-run-a-host-on-sia-2159ebc4725.
Follow the hosting handbook at http://forum.sia.tech/topic/1037/the-hosting-handbook to learn more about how to be a good host and maximize profit.
Learn more about hosting at our Wiki: https://siawiki.tech/host/hosting.