It is likely the A axis is a motor that is slaved to X or Y for the gantry (if it is a gantry router? ) so only 3 axis machine as A will not be a true axis.
You can change the way the arrows work from the Config menu then System Hotkeys.
Convention is X left to right, Y from and away to you and Z up/Down.
X- is with tool going left in relation to work X+ going right
Y- is tool towards you Y+ tool away from you
Z- Tool down Z+ Tool up.
For some basics use the wizards and mess around with them to see how things work, just keep the E-Stop handy

Hood