There are huge amount of antivirus programs to select from. On what basis would you buy one? Are you using the one which you got with your system? For that matter how do you know that it is effective? Normally test organization researching on antivirus and rank products every month on performance, effectiveness and usability.
Check whether you want to spend money or not. Most antivirus support programs are paid solutions which would cost some money. There are free products available too. Free products also have same detection features so even they can identify as well as block malware properly. However the paid or licensed version has certain extra features and function as a complete security suite. You would get additional features like an anti-theft tracking, additional firewall, phishing filters, anti-spam features and so on.
However the free ones would nag you. They would display offers encouraging you to pay. Basically the free one does marketing for the paid one. You need not pay any subscription fees. You should use the product from a reputed company. The antivirus product would be subject to lot of scrutiny and would appear in the tests thereby giving you some idea of how it performs. There are some which disguise themselves as such programs. These types of malware pretend to be a legitimate product but is malicious and doing horrible things in the background. If you choose an unknown virus it would put you at a risk of getting infected with bug.
Normally such tests are ranked on basis of usability, protection, performance and cost. This would reveal how much friendly is the user interface, how successful they were at blocking samples and how well they performed while doing so.
Choose an antivirus support which keeps the security at the highest. No application is perfect and seldom might a malware enter. In that case as a user you should be highly alert and practice things in order to keep the system more secure. Reduce the system exposure to malware.
You can narrow your options based on what operating system you use. If you plan to upgrade in the near future then consider that as well.
Do analyze how you use the system? If you are a web surfer or a gamer then you would require protection accordingly. For instance a gamer would prefer software which recognizes when media or games are played as well as suppresses unnecessary activities. Those who have old system they would be worried about performance as well as impact as well as use the one which support older operating system.
After meeting the usage and operating system needs check certification as well as testing agencies.