The best way to secure your environment is to make sure you never exchange data. The best way to avoid exchanging data, is to cut your data line.
Because this is not a practical advice, what do we need to focus on, when mitigating security risks? The first step is to build some knowledge and experience on the topic. Following best practices and broad accepted guidelines. Common sense is also a good rule of thumb.
This course will not turn you into a security professional, but it will help you on the road becomming a professional with a good sense of security.