How do I secure my wireless internet?

My computer is running slow and I have been told it is due to my unsecured internet. I have been meaning to secure for some time but do not know how. The best I could do was to go to manage networks and change security settings to WPA-2. I thought this would work but the computer still says it is unsecured so obviously I was wrong. Ive tried to search the internet but all the websites are too wordy and turn out to be answers to different problems. Step by step would be nice and all help is greatly appreciated. Thank You.