How do i add a /setCoins and /addCoins command?

You can use the Chatted event to detect when a player chats something, then you can use string.find() and string.split() to determine if it is a command, and what value they want to set/add. This would be a server script

Here are the links to the wikis:

https://developer.roblox.com/en-us/api-reference/event/Player/Chatted

https://developer.roblox.com/en-us/api-reference/lua-docs/string

Here is a sample script

-- lets assume that someone has said "/setCoins 50"
-- and assuming that setCoins means to set the coins, while addCoins is adding to the value

coins = player.leaderstats.Coins -- where are the coins located?
msg = "/setCoins 50" -- you would put a variable for the message inside the parentheses normally
--                             v right here
player.Chatted:Connect(function()
	if string.find(msg,"/setCoins") then
		local value = string.split(msg," ")[2] -- gets the second string that is separated by a space in the msg
		coins.Value = value -- aka setting
	elseif string.find(msg,"/addCoins") then
		local value = string.split(msg," ")[2]
		coins.Value = coins.Value + value -- aka adding
	end
end)

Hope this helps