Yahoo Chat Where and How?

Okay here's the scoop. I used to be on AOL. I changed now to yahoo. The problem is it was very easy to chat or instant message on AOL. I can't figure it out here on yahoo. I suppose it might be very easy on here if I just knew. Another thing strange has been happening. When I am opening my email I will often get a message saying '; You've been signed out of chat because you signed in through another device or computer '; Huh ? I don't have another device such as a cell phone or another computer. This computer I am using is the only way I have of getting on the internet. Yet I keep getting this message. I hope it isn't someone who has wireless tapping into my computer. Anyways someone please tell me how to use chat on here and also why am I getting this message.