1. Google's Chromecast
Google's Chromecast is, in my opinion (I have one), the easiest convenient way to stream videos online from pretty much any website. The Chromecas connects to your TV's HDMI port and gets power from the TV's USB port. So your TV needs to have both an HDMI and a USB port in order for this to work. It connects wirelessly to your computer - through wi-fi. Works great with Windows or Apple computers. The price it between $30 and $50 (depends on deals). Once you buy it, the instructions to set it up are very easy to follow. Click to see current price at Amazon.
2. Apple TV
Apple TV is very similar to the Chromecast. It connects to your TV into the HDMI port, however it requires power from a wall outlet, not from a USB port. It connects wirelessly to your wi-fi (and computer), however it also comes with an Ethernet port (for an internet cable, simply put). People recommend it mostly for use with Apple computers, not for Windows, especially since it's a little bit of a hassle to get it to work with a Windows PC. The price is between $70 and $100. Click here for pricing at Amazon.
3. VGA or HDMI Cable
This is the cheapest option and I still use it sometimes. If you have a laptop, you could plug it directly into your TV. All you need to know is whether your laptop (and TV) has a VGA or HDMI port. Most modern laptops (and TVs) have HDMI ports while most old laptops may have a VGA port. Here's how they look.
You may need to modify your display settings once you plug the TV to the laptop. The way in which this is done, and the number of steps required, will vary depending on your operating system. Even though I have a Chromecast I still use a VGA cable from time to time, especially if I want to play a DVD on my laptop and watch it on my TV (nope, don't have a separate DVD player). The price of both cables is about $6 for 6 or 10 feet. The price increases if you need a longer cable. Check out the VGA cables at Amazon or HDMI cables at Amazon.
Please note, this information was last updated on 03/31/2015. Prices and product availability will have changed if you're reading this from the future ;)