How can I put password for viewing particular website?

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I want to put password on my ptc site in my computer. The computer Im using right now is for everyone in the house but I dont know how? By next months im gonna use another computer. Im worried that someone in the house might find out about the ptc sites and if they join using this computer. Our both accounts will be terminated. That's why I want to put password before they can view the ptc websites that I joined in. Like example if i search on the address bar the site of mylot, there is a msg saying Im not allowed to view the site and I have to put passwaord to acces the site. Pls help me. Tnx