First of all, determine why you missed your flight. This should be quite obvious, but the specific reason will impact how your airline responds - and how much you have to pay (if anything) to board a subsequent flight.
If your passport or ID has expired, for example, this is down to you. The airline will probably let you re-schedule at a later date for a fee, plus any fare difference. Of course, obtaining a new passport or ID might mean you aren't able to leave until weeks later.
More common reasons for missing a flight include reading your ticket time/date wrong, or over sleeping. In such cases, you should head to the airport ready to go and explain everything to your airline's customer service representative. It's just possible they will rebook you onto the next available flight - perhaps free of charge or for a fee. However, some airlines will ask you to buy another ticket entirely. Being polite and patient will help you get the best outcome.
If you missed your flight for reasons outside your control, then you may have a better chance of being rebooked. You may have been delayed by an accident en route to the airport, for instance, or you may have been impacted by bad weather or strike action. Your airline may be sympathetic to your situation and speed you on to the next available flight.