Can someone answer a hacking question please?

I woke up this morning and had a tweet from a very evil user on twitter. I tweeted him back and blocked him. Shortly thereafter, a Y/A contact I chat with sometimes in Yahoo Mail told me that I had sent him a link for pics to look at. Was not me. He told me to change my passwords and I did. How concerned should I be now and do you have any suggestions for me as far as security while on my computer? I'm not a techie, so please keep it simple if you can. I appreciate your answers. Thanks!