How do I know if my computer has been hacked?

[bluest]

Hello everyone!

I was playing this game yesterday. And we had an argument. We were talking about the system and stuff. There were 2 hackers in the room, well I don't really know them. I've never talked with them before. So it was my first time. Well basically, my opinion was different than their opinion. I didn't really think that they could go this far? But well... CMD (Command Prompt) popped up randomly. And then this text came up. "*my opinion here*, huh?". Seems like they just didn't like my opinion. And decided to hack into my computer or what?

No I didn't click on anything. It just randomly popped up. Which made me get worried. So I looked it up and found a video about how to get into someone else's computer with CMD stuff... I have no idea how that works. And I don't want to know. I guess that's how he did it?

After that, I called my friend and told her what's just happened. We laughed. Everything was good... But then I got a notifation from my iCloud. "You've entered an incorrect password..." I wasn't even doing anything. So I checked the hacker's Twitter. I found out that he tries to guess the passwords. And then he gets their pictures... Leaks them on his Twitter.

And my computer lags sometimes... I don't know why. It didn't happen before. So how do I know if he's spying on me for fun?

--I know my English isn't that good, but I tried my best.

 

[RolandJS]

My first educated guess at a first solution set:
-- cease all communications and game-playing with these two alleged hackers
-- cease all game-playing and chat-rooming at that particular and all such online sites, for now
-- cease all communications with anyone you do not really know as a trusted friend on social media, for now
-- if you are not adept at computer security and/or anti-virus/anti-malware-fighting, find a local computer guru that you trust, and let him/her guide you through acquiring, installing, shields-setup, running scans, to eliminate any foreign utility or program that can even remotely carry on viri and/or malware [think "Darth Vadar/Death Star" activity] on and/or from within your computer.

Here is a solution set that must be done TODAY, TOMORROW, this week:
Create an old-fashioned notebook [that will reside in a locked drawer or lockbox when not in use], grab a pen and pencil, begin exact and accurate printing of ALL usernames and passwords of any kind
THEN, begin the process of changing your passwords to strange un-easily-guessable combinations of capitol and small letters, numbers, no spaces for each and every account beginning with your Windows, your email accounts [!email acct passwords MUST be changed immediately!], and continue until all of your accounts have much stronger, much longer, much more secure passwords than ever before.

Have you been making any kind of routine, current, backups of your OS and data? If not, have the above-mentioned computer guru walk you through the acquiring, installing, using on a routine basis a well-known, well-respected backup/restore program [this includes making usb or dvd boot] -- which you will begin using after the computer has been cleaned of any/all viri/malware programs and activities.

I post the above because I have absolutely no idea of what is installed and running on your computer; I have no idea what web sites, online games, social media interactions & swaps, etc. that has been and still is going on. Only guesses for now.