The nameplate is split into four parts, all with different values you can change. These parts are

CHAT - This is your nameplate that shows up tied to your messages in the game chat.

LIST - This is your nameplate in the tab list

ENTITY - This is your nameplate above your head

ALL - This is for all of the above

All

ALL has a single function, setText

nameplate.ALL:setText("Me")

Chat

CHAT only has two functions: getText and setText

nameplate.CHAT:setText("Me")
nameplate.CHAT:getText()

nameplate.ALL:getText() only gets the nameplate changes made by setText, so if you haven't previously set the text, it will return nil. This is true for all permutations of getText

List

CHAT and LIST have the same functions

nameplate.LIST:setText("Me")
nameplate.LIST:getText()

Entity

The entity nameplate has far more functions, they are all accessed via ENTITY, here are some examples

nameplate.ENTITY:setText("Me")
nameplate.ENTITY:setPos(0,1,0)
nameplate.ENTITY:setShadow(true)

setText(string)

Sets the text of the nameplate

getText

Gets the text of the nameplate as set by setText

setVisible(bool)

Sets the visibility of the nameplate

isVisible

Gets the visibility of the nameplate

setPos(vec3)

Sets the position of the nameplate in blocks, this is an offset of the pivot location so 0,0,0 is the pivot's location

getPos

Gets the position of the nameplate

setScale(vec3)

Sets the scale of the nameplate

getScale

Gets the scale of the nameplate

setPivot(vec3)

Sets the pivot of the nameplate

getPivot

Gets the pivot of the nameplate

setLight(vec2)

Sets the brightness of the nameplate text, the first arg is blockLight and the second arg is skyLight

getLight

Gets the brightness of the nameplate text as set by setLight

setOutline(bool)

Sets if the nameplate text should be outlined. Incompatible with setShadow

hasOutline

Gets if the nameplate has an outline

setOutlineColor(vec3)

Sets the color used for the outline in rgb format and 0-1 ex: (100/255,100/255,100/255)

setBackgroundColor(vec4)

Sets the color of the nameplate background, in rgba format and 0-1 ex: (100/255,100/255,100/255,100/255)

getBackgroundColor

Gets the color of the nameplate background as set by setBackgroundColor

setShadow(bool)

Sets if the nameplate text has a shadow

hasShadow

Gets if the nameplate has a shadow

Using JSON

If you wish to change the properties of the text outside of just the text, you can use json to change things like color, boldness, or any other properties you can give text in Minecraft, including via the color codes. You can also use a json text generator from anywhere online.

nameplate.ENTITY:setText('[{"color":"red","text":"Me"},{"text":"!","color":"#09ff71"}]')

Here's an example, you must wrap all the json given in apostrophes (') like in this example. A json generator will not come with the apostrophes around it, you need to add those yourself