Read This! It will Help you!
ok, in a trigger of initialization, you have to create a dialog box, and variable it, then you have to make buttons and variable them too,(you should use arrays, its easier) then, in another trigger, you can check if the player pressed a button(which is set as a variable) and make the mode set, which i will tell you later.
you will still need the trigger at the beginning like game time is 1 second, then show dialog box(set as variable) for player 1(red) or whom ever picks the game mode.
Next, you need separate triggers to determine how many kills you need to win
so each trigger you will start off with
"Unit - Generic unit dies"
Triggering unit is a hero = true (in the boolean section)
For this action however, you will need to set a variable(namely Kills, integer variable) and then in the actions, above
set (variable, Kills) = (variable, Kills + 1)
and after that you add an If Then Else trigger
If (variable, Kills) = (ok game mode one, so lets say, 20)
Then (make victory for all players allied to the killing player and the killing player)
Else (leave that blank)
Other gamemodes, just copy the trigger, and change If (variable, Kills) = (this number to 40, 60, etc. all the way for the number of game modes)
For every Gamemode trigger, go to the top of the trigger, not way top, but you will see 2 boxes that are checked, uncheck the one that says "initially on"
In those triggers with the button pressing,(which the player presses button(variable) trigger) then in the actions, you create all the pudge(or w.e) for the players, you know yadda yadda set up the map, then add the trigger, Turn on trigger and turn on the game mode that the button is reffering to. So if this trigger is for Game mode 1(the pressed button is for game mode 1), you would turn on the Game mode 1 trigger(that is initially off) to On.
Ok, im sure that should wrap it up, if you need any questions ask.