Tables Help? Please?

Hey there guys, I'm new with tables and need some help. My first problem is, how do I get a string, number or bool from a table, for example:

I have this table (I'll use throughout this question): table = {23451,23456,34567,45678,56789,true,false,"Random","random"}

I want to pick out a random item from the table, would it look something like this: a = table[math.random,1]

The second thing is, how do I remove something from a table? here's an example:

A player leaves the game, I want his UserId to be removed from the table. Would it look something like this:

game.Players.PlayerRemoved:connect(function(player)
	for i,v in pairs do
		if player.userId == v then
			--How do I remove the number from the table??
		end
	end
end

I also want to know how I can add items into a table, for example, a player touches a door, I want his user ID to be in the table:

script.Parent.Touched:connect(function(player)
	for i,v in pairs do
		if player.userId ~= #table then --If the player's userId is not in the table
			--Help here?? Inserting the player's userId into the table
		end
	end
end

Thanks for your help!