Avoid Sketchy Websites
The #1 way to avoid getting adware or other malware is to be careful about the websites you visit. For instance, sites that let you watch videos or play games for free or download free software are notorious sources of malware.
Enable AdBlock Filter Lists
AdBlock includes a filter list called Malware protection that can block downloads from sites that are known to host malware. This filter list should be turned on by default when you install AdBlock. You should confirm it's enabled, just in case. You may also want to enable Easy Privacy for an added layer of security. Find out more about AdBlock filter lists here.
Anti-Virus and Anti-Malware
Always use a reputable antivirus program. You can find reviews and recommendations for good antivirus programs using your favorite search engine. In addition, keep an anti-malware program installed to scan your computer when you think you might have picked up malware.