Before you test your speed, close any other browser tabs. Also, stop any downloads you may have in progress. You will want to use a wired Ethernet connection instead of Wi-Fi, just in case your Wi-Fi is spotty.
Speedtest will also recommend you download Chrome to improve your speed, but it isn't mandatory. Whichever browser you use, you'll want to update to the latest version. Not only will your speed improve, you'll be less susceptible to viruses and malware.