No left click, keyboard misbehaving

Hello all. I have an iMac 2011 running Yosemite 10.10.5

Since purchasing it second hand the mouse has never worked as I expected. For example when I click on apps in finder it acts as though it's a right click. I get the options of open in new window etc but I can't click on anything.

Sometimes I can click on an item with my left click, sometimes it wants right, sometimes not at all. Today my keyboard has decided to do some shortcuts I don't want. I'm trying to update software, but my Apple ID has a 'b' in it. When I hit b it goes back, not types the letter.

I've tried reboot PRAM, I've made sure the left click is chosen in mouse settings, I don't know what else to do.

Any help appreciated

It's a corded mouse. But it happens with all mice

Suggest you upgrade your operating system. Yosemite is no longer supported with Security or other updates. Download Sierra, burn to a thiumb drive using DiskmakerX, boot from the thumb drive and erase and format the hard drive and do a clean install.

Make sure you have a current backup first.