You have to decide first what kind of display you want. Simplest are probably 7 segment displays which are usually a set of LEDs you use to display Digits or even Characters. These you can drive each led by connecting it to any micro digital output. Then you have alpha numeric LCD displays. These are black and white and can show one or more lines of letters and numbers. These tend to have a lot of pins and require a dedicated IC to drive them. You can buy modules that have the driver IC built in and then you connect it to your micro via some serial protocol like I2C or SPI or even uart or some other interface. You should identify a particular display you want to use and then look at the datasheet/manual for details on what kind of interface it requires.

Some mbed devboards even come with an LCD display and there is example code as well (disco L476VG for example). So that is another route to take. You could also search mbed libraries to see if there is a popular LCD driver that you could use. For example this one seems to be hugely popular and targets a particular interface IC - so if you got an LCD with that module it should work: https://os.mbed.com/users/simon/code/TextLCD/