Q: MacBook Pro develops new quirks daily that further limit usefulness

I'm very computer illiterate.  I just purchased a MacBook Pro laptop a few months ago.  I'm recently retired & intended to teach myself how to use the Mac. 

Specs:  OS X, Version 10.9.5; Processor 2.5 GHz Intel Core 15;  Memory 4 GB 1600 MHz DDR3; 13 inch mid 2012

I don't know what I've done to it.  I've downloaded a few APPs.  Then I began to notice it getting slower.  Next, when it would first boot up a box popped up that said, "You've entered the wrong password for iCloud. Enter the correct password for (my user name for Apple (help?).  As the days passed more boxes began to pop up asking for other passwords (for email e.g.).  Then it got to where whatever topic i was asking for information on, the sites that came had nothing to do with that topic.  I can only log into one email account.  The other accounts say that I've entered the wrong password. 

Can anyone help me?  If anyone knows of online course(s), book(s), manual(s), or online videos that can help me learn more about the Mac.  Any advice by chat from you surfers will also be appreciated.

I have only used it for surfing the web.  I tried to download some books but failed.

Thank you,

Zeb