I have to make a program with that converts metrics to feet, inch ect. I have no idea how to even begin doing this. Theres two combo boxes one for the units of metrics and the user chooses millimeter, centimeter, meters, and kilometers. They enter an amount to convert and then thers another combo box to choose what US units they want to convert to such as foot, inch, miles, yards. I just do know how to do this having two combo boxes it seems like there could be a simple way to do this but I really dont know how to even start ive been looking at a few ideas saying to use if else and I'm really just drawing a blank if anyone can give me any input on how to begin I would really appreciate it!

Combo Box? Do you have to make a gui for your program?