Warning: require(vendor/autoload.php): failed to open stream: No such file or directory #11

I am new to sendgrid. I have the API Key, and now I am trying to use for php. I have downloaded the code directory but now It is giving me the below warning.

Warning: require(vendor/autoload.php): failed to open stream: No such file or directory

I have no idea about how to use composer (already installed). Help me to sort out this issue please.