To install cURL you can type a simple command in the terminal. cURL can be installed with apt-get, so all you would have to do is to type the below in a terminal window:
sudo apt-get install php5-curl
The sudo part of the above gives you the necessary administrative privileges, using apt-get without sudo might result in an error.
After installing cURL you should also restart Apache to make it work.
sudo service apache2 restart
If cURL is not installed
A common error message when cURL is not installed on the server is something like the below:
Call to undefined function curl_init() in blah/blah/blah on line xx
The solution is to install cURL using the terminal, as mentioned in the top of this article.