@RoyPenrod

First, if you've not already done so, I would recommend running:

sudo apt-get update && sudo apt-get upgrade -y

This will update/synchronize the OS repositories for your release and then upgrade all existing packages on the server that need to be. This ensures that you're running the latest stable packages and that any patches or security fixes that have been released since the OS image was created (by DigitalOcean) are applied.

I'll normally run the above command each time I login and before I install anything simply to ensure everything is as up to date as possible.

--

You can use the following command to install sendmail:

sudo apt-get install sendmail -y

Once installed, simply run each line (from my comment above) as an individual command via the CLI, starting with:

export SMAIL=$(which sendmail)

(Hit Enter/Return to Execute)

Then:

echo "" >> ~/.bashrc

(Hit Enter/Return to Execute)

Followed by:

echo "export $SMAIL:$PATH" >> ~/.bashrc

(Hit Enter/Return to Execute)

and finally:

source ~/.bashrc

(Hit Enter/Return to Execute)

That should take care of sendmail for you :-). The below is a few basics to help you with software management.

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When it comes to software management, you'll use a series of commands.

sudo apt-get install package-name

... to install various software packages - specifically "package-name" in this example (similar to the above where we installed sendmail).

sudo apt-get update

... to synchronize system packages with the distro's repositories

sudo apt-get upgrade

... to upgrade either a specific package or all packages; running the above command with only -y as an argument means upgrade all available packages while specifying a package, such as php5-fpm (as an example) will upgrade that package and it's dependencies, but nothing else.

sudo apt-get remove package-name

... to remove a package named "package-name" and its dependencies, though configuration will remain in place - to fully remove a package, you'll need to run the next command.

sudo apt-get purge package-name

... to completely remove a package named "package-name" (files, directories, configuration etc) - there's no recovery option when using this command, so only use it when you are 100% sure that you want to remove a package entirely.

sudo apt-cache search package-name

... allows you to search through all available packages for a package named "package-name" - if no package exists, nothing will be returned - if a package does exist, the package name shown on the left is what you'll use in place of "package-name" if you want to install it.

sudo apt-cache search package-*

... note the asterisk - it functions as a WildCard, meaning that the results returned will be based on "package-" instead of "package-name" - the WildCard can be used in the other commands as well (specifically install, remove and purge), though I limit it's use primarily to searching due to the reason outlined for the purge command - this means that a search on "php5-" will return all packages that begin with "php5-" (i.e. php5-cli, php5-fpm, php5-json, php5-mcrypt etc).

sudo apt-cache search package-name --names-only

... this functions similar to the WildCard, except you're not setting a WildCard - instead of using "php5-", we can run this command on "php5" (without the - or *) and get the same results.