problems with signing in with twitter on my laptop?

[Caroline]

I just got this laptop working today-its my sisters old laptop and i mean its really old!! its a dell inspiron 1100, idk if that helps? she was having problems with it randomly shutting off and the motherboard, but anyways its up and running so i download anti virus software and my wifi connector i use on my computer, i tried logging in twitter numerous times. I can get to the website and put in my information but when i hit enter it says like the site's forbidden. its a red screen with a white box that has the text inside and a yellow triangle with a ! on the inside of it. I'm using google chrome, but i also tried with internet explorer, and it wouldnt connect. i read more about it and it says it needs a third party to trust the website or something? what should i do so i can log on? what does this mean? Thanks for all the answers!