Assuming you are using the Safari web browser, do this:
"you can reset Safari totally (empty cache, forms, passwords, cookies, etc) by using the Reset Safari... option in the Safari menu.
If you're just after cookies, look at Safari preferences -> Security -> Show Cookies.
If you're after stored passwords, look in Keychain Access (in /Applications/Utilities). You'll find them under Passwords (as Web form passwords)"...