For many people, there are simple tricks that can dramatically improve their security posture. These either remove the common entry points for attackers, or add additional barriers against their attackers. Combined, they are the easiest technical methods available.
The first, and most basic of options, is to ALWAYS have the current version of all software you use installed. If you are on Windows, Microsoft has a regular schedule for releasing updates. You should even have automatic updates enabled if possible. Adobe also follows this schedule, and Reader updates should be just as important as those Microsoft patches. Same goes for Apple and OSX updates, and the maintainers of various Linux distributions, although the tools to do these updates vary. Check in your system documentation.
Secondly, remove the software you do not use. If you cannot name the reason why you have Java installed, for instance, uninstall it. All it does is add a major vector for attacks. This goes for any software. If you’re not using it, remove it.
Thirdly, be sure to install and configure a firewall. This is the last barrier against outside attackers, and you want to be sure you are only letting in what you intend to. Create as narrow of exceptions as possible, and look for configuration suggestions for your use case. Remember you don’t need to open ports to browse the internet, unless you are blocking inbound and outbound traffic.
Forth, have good antivirus installed. I honestly find this less important than the first three, just because reducing your exposed surface ends up being far more useful. While anti-malware software and firewalls definitely should be exposed, they can and will miss things. There are plenty of free options for antivirus, although I usually direct people to AVG on Windows. Microsoft Security Essentials is also a decent choice. No matter your operating system, however, you need to have something installed. With cross-platform malware such as java or pdf-based exploits, and growing desire for mobile attacks, assume that there is something out there.
Fifth, have good password security. Use a strong password to log into your system. Do not use an account with administrative access for everyday activity, to prevent malware from gaining a strong foothold if it gets onto your system. I’ve talked about this previously, but also use a password manager. Generate unique passwords for every single site, it protects you if one of those sites are cracked. Remember if you use the same password everywhere, your bank security depends on the security of that crappy forum you joined.
Finally, where it is available use use two-factor authentication. This is far from foolproof, and there are ways around it, but it stops the cheap attacks used to gain access to your email, your bank, social networking, and more. It also typically protects against repeated incursion on those accounts.
The combination of all of these tools creates an multi-layer protection that is far better than most people have. If any of these are missing, it creates a hole that can be exploited. Together, you will find yourself avoiding many of the greatest pitfalls and be far ahead of others.