Most people use ClamAV for their server anti virus, some still question the need to have antivirus on their servers. On that point, its true there is very few virus / malware out there for Linux. If it comes from your server and the user is running Windows for example it can infect them. Its also your reputation to think about.
Although Sophos claim 1GB as the minimum RAM requirements, it does run on a 512mb Digital Ocean droplet without issue.
To get started go to here to download https://www.sophos.com/en-us/products/free-tools/sophos-antivirus-for-linux.aspx
1) Extract the file you download and upload the folder to your server using SFTP.
2) Connect to your server over SSH and elevate to root.
3) Change to the sophos folder using
4) Run (with root privileges)
5) Sophos AV will install, follow the prompts ensuring you select the free option
If you have a supported operating system that Sophos provides pre compiled kernal modules for you are finished. If you don't have a OS that has pre compiled binaries like myself who uses Debian follow these steps:
If you don't have a standard OS supported by Sophos
Again ensure you are running with root privileges:
1) Ensure make is installed with
apt-get install make
2) Ensure the extra header packages are installed to allow the custom module to compile correctly
apt-get install linux-headers-$(uname -r)
3) Install the modules and let it compile by running:
4) Restart the service using