What not How
I think the first step to understanding computers is learning what you can do.
Then you can decide if you want to do it and how to do it.
This page is more "what to do" rather than "how to" or "why you should do it".
Feel free to ask me questions.
If you do nothing, they will come.
Computers go bad by default.
You will accumulate shortcuts, cookies, homepages, browser toolbars, start programs, TSRs, bloatware, malware...
And some of these things feed on each other and form a hideous symbiotic relationship and botnets and stuff.
Your computer and internet will slow to a crawl.
Don't trust your computer.
Computer security is so complex, you will prolly get hacked.
Don't keep your credit card online.
Use PayPal instead of credit card.
Log out after you're done on a site.
Don't let your browser or operating system remember (save) your passwords.
Don't keep your passwords on your computer.
Don't login to stuff at a public wi-fi hotspot.
Disconnect when not using the internet.
Don't use Internet Explorer.
Use the strongest passwords you can.
If you don't use a password often enough to memorize it...
...make it random and long instead of something easy to memorize, and keep it in a password manager.
Protect your email.
1. You use your email as a username on a site.
2. Someone hacks your email.
3. The hacker can reset your password on the site where you use the email as a username.
Don't trust an email you get from someone you know.
An infected email account can email its contacts.
A clickable link can say anything.
The link may say amazon.com...
...but will take you to badsite.com.
Don't click shortened links like http://bit.ly/GVBQJS...
...before checking them at http://unshorten.it/ or a similar site.
The .com part is the important part.
Keep your operating system and software up-to-date.
If you don't need a software, get rid of it.
Don't download torrents.
Back stuff up.
Scan for malware weekly.
I use(d) free versions of these:
ScriptBlock (script blocker for Chrome)
Vanilla Cookie Manager (cookie blocker for Chrome)
Web of Trust (website ratings)
Adblock (ad blocker for Chrome)
noScript (script blocker for Firefox)
CookieShield (cookie blocker for Firefox)
Adblock Plus (ad blocker)
Web of Trust
password manager (There's lots of password managers available)
Consumer Reports on security: