|If you want to send SMS messages from a PC which is connected to the internet, you have to go to a website which provides that service. Very often your own mobile network provider's website provides that option when you log in at their website, so try that first (you'll have to register with your mobile number first to prove that you are one of their customers).|
Alternatively, use a third-party website which provides an SMS service. Which website you use depends on which country you are sending from, but in the UK try one of these:
If you are not in the UK, search for Free SMS using your local Google Search website.