How to search for a space in chat?

String patterns are just what you need. I recommend splitting the string after the signal character(s) (in your case, the ";;") into a command and argument.

local SignalCharacters = ";;"
local Message = ";;setHealth Username 100"

function HandleMessage(Message)
    if Message:sub(1, #SignalCharacters) ~= SignalCharacters then
        -- Return early if message isn't a command
        return
    end

    local CommandSubstring = Message:sub(#SignalCharacters + 1)

    local Command = nil
    local Arguments = {}

    for Match in CommandSubstring:gmatch("%w+") do
        -- If there's not a first match yet, it's the command!
        if  Command then
            table.insert(Arguments, Match)
        else
            -- If there's not a first match yet, it's the command!
            Command = Match
        end
    end

    if not Command then
        return
    end

    -- Continue here to check if it's a valid command, and has the valid number of arguments, et cetera.
end

HandleMessage(Message)