Find a couple of basic example projects 1 using a 20x4 lcd display, and another that reads serial input. Python projects and examples would probably be the easiest to find and follow.
Ypu may find something VERY close to what you want too
With 2 x example projects you can study and learn how the programs work and use the h/w or input that you are interested in. After a couple of kettle boils and cuppas you should understand the code enough to start hacking the 2 projects together to create something that fits your requirements.
If you struggle to follow or understand the examples, or hit a hurdle with the mashup come back with specific Q's
a couple of projects from the top of the google results when searching using basic keywords
(20x4 lcd Pi)
(serial input Pi)
https://www.instructables.com/id/Read-a ... pberry-Pi/