Macintosh computers are the most secure around. They have the amazing abilities to repel internet attacks, and stay very secure against physical hackers. There are very few viruses that work on them, and very few ways to take control of one. Although they are very safe, sometimes you need to help them along, to get them secure.
Things You Will NeedGood password
The first thing you need to do is make sure your built in firewall is active. Go to System preferences, Security, Firewall (tab) and make sure the green light is on. If the light is red, click to turn it on. If you unable to click check to make sure the lock icon at the bottom of the window is "unlocked". If it's locked, you may have to put in your administrator password to unlock it. Once it is unlocked you can change the color of the light (or switch the on/off switch located on that same screen). Enabling this firewall protects you against attacks from the internet through your airport connection.
The second thing to do is make sure you have a strong password. Many websites can describe how to make an effectively strong password. My advice is to make one using codes from activities or interests. If you like chemistry make a password out of chemical compounds (ex: feh2o), or golf (Holein1). These codes usually contain letters and numbers so that makes them very strong.
There is no use in having a good password if you have left loopholes in when the password is needed. Go to system preferences, accounts, and set yourself a password. Then go to system preferences, security and in the main tab change the settings so the password is required after screensaver and sleep. Also, always make sure that automatic login is disabled.
If you require extra security because you have sensitive information like bank info, or business stuff on your Mac, enable "Filevault". Filevault is an encryption system that locks down everything in your personal Home folder. It encrypts everything will 128-bit AES encryption which is the government standard for security. The computer encrypts and de-encrypts files as you use them, and rarely will you ever experience a speed lag.
To enable Filevault, go to system preferences, security, and the Filevault tab. Before enabling Filevault, make sure you have double the amount of space you are currently using on the hard drive, as free space (Filevault copies everything while encrypting it and then deletes the other copy). You may have to unlock the lock icon at the bottom of that window to enable Filevault. Set your master password (WARNING: if you forget this password, you are fishing for the laptop in a creek, with no paddle or rod or net to speak of. DON'T lose the master password. Apple is very serious about its encryption and there will be no way for you to get your data back. NO WAY) to something secure and let File vault do its thing.
WARNING: enabling Filevault will take a long time depending on how much data you have on your hard drive. What takes even longer is securing erasing the hard drive after encryption. Filevault basically copies all your data while encrypting it, and then deletes the other copy. Many data recovery programs can recover that data because your computer doesn't really "get rid of it" it merely says it's not there, and then writes over top of it when you "ask it to". Securely erasing the other copy (and the same with securing erasing your trash bin) tells the computer to remove the data and then write over top of it so it can't be recovered. Let Filevault sit for as long as it needs (130Gb took 2 days) and do not interrupt it. Don't unplug it, don't close the lid, and don't do anything to it. Although it takes a while to begin with, it will be well worth it.
The last important thing to do is to install an anti-virus system. Many companies down make their programs for Mac, but Norton (by Symantec) does. It costs a bit, but might be well worth it. If you don't want to buy it, don't worry, apple is very secure without it.
The last thing to do is always make sure you are installing updates. Apple automatically downloads and installs most security updates in the background, but make sure you go to the Apple icon at the top left of your menu bar, and select updates and install once they've checked your system.
Macs are generally very secure computers. Makes sure your password is healthy and strong, and your firewall is on, those are the most basic things to do!