A compilation of useful resources that are needed in order to start automating your home using smart devices.
Most of the stuff here are pretty cheap and Home Assistant is a perfect app that can combine all of those devices into a single dashboard app.
Raspberry pi 2 or 3 ($20-$50) Recommended but not mandatory. You can run home assistant on any PC or laptop, but its much easier to install it on raspberry and place it somewhere in home and forget about it
Xiaomi Gateway ($30) Main "hub" if you opt into buying Xiaomi smart devices (switches, lights, sensors etc). It supports all Xiaomi, Aqara, Mijia and Yi products. You can also buy Aqara Gateway which is Apple Homekit compatible and should work with devices that original Xiaomi Gateway supports. You need "Mi Home" app in order to set gateway and other Xiaomi devices. Xiaomi Gateway connects to your Wifi and all other Xiaomi smart devices connect to gateway using Zigbee protocol
Xiaomi and Aqara smart devices (around $10-$30 each). There are a bunch of cheap sensors like temperature+humidity sensor, window/door sensor, natural gas sensor, smart switches, smart power sockets and a bunch of other stuff which can be found on ebay, gearbest or aliexpress by searching for "xiaomi smart" keywords.
Yeelight lights (~$20 for RGB bulbs, $30-$40 for led strips) These are very good (and cheap) alternative to Philips Hue. Yeelight has its own dedicated application which can be used for automation, but it can be discovered by Home Assistant if you turn on LAN control in Yeelight application. Yeelight devices don't require any kind of hub because each device connects to your Wifi (only 2.4GHz, 5.2GHz is not supported).
Philips hue (~$40 for RGB bulbs, ~$30 for dimmers, $100+ for kits with gateway) Currently one of the best smart lights you can buy, but pretty expensive. Lights communicate with each other using Zigbee, and they require Philips Hue hub which connects to your network using LAN cable.