Fly! is the button you click in the Client to start a flight.
The first page you will see is the refuel page where you can set the fuel for your flight.
Important note: Some add-on aicraft have their own refuel module and prevent the Client from setting the fuel (for example the PMDG aircraft). In this case you have set the tanks using the add-on module and then just click on Fuel up! on the refuel page.
If you select the simple fuel mode in the client options you will see one box where you can enter how much fuel you want for the flight.
In detailed fuel mode you will see one box for each tank and can set the tanks individually.
Having clicked on Fuel up! you will get to the confirmation page where you can check the flight data again before starting the flight. If with your current fuel, pax and cargo the maximum take off weight is exceeded, you will find two change buttons next to the pax and cargo values. By clicking on one of them the pax/cargo value will be reduced to a number where the whole aircraft is within the limits.
Finally you can start your flight and get to the Fly page where you can see some data about your current flight status.