I need a code that when I send a letter to the serial port from my PC, it takes the code and makes an equation (a=hello,b=bye,c=idk) than prints the result to my 16-pin LCD screen.
If someone could help me, It would be great
What have you tried? What were the results?
The serial input basics tutorial may be a starting point?
Have you got the lcd screen working, just to print a "hello world" type message as a test? If not, start there 
Then have a look at example 1 here (edit: same link as previous post), and instead of just printing the character out, use it in an "if" and then output the appropriate text.
Show us a good schematic of your circuit.
Show us a good image of your wiring.
Give links to components.
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Well, I figured out how to make it work. But thanks to everyone that tried to help me
Gklp:
Well, I figured out how to make it work. But thanks to everyone that tried to help me
That's great, but for those of us who read the forum to pick up tips and learn from others, how about posting your solution and giving a little back?
GM