Changing Clothing at the start of the game?

If you want to do this the easiest thing to do is to use the PlayerAdded event.

game.Players.PlayerAdded:connect(function(player) 

Next, check to see if the player has a shirt or pants

game.Players.PlayerAdded:connect(function(player)
    if player:FindFirstChild("Shirt") then
        player:FindFirstChild("Shirt").ShirtTemplate = "[insert shirt asset here]"
    end
    if player:FindFirstChild("Pants") then
        player:FindFirstChild("Pants").PantsTemplate = "[insert pants asset here]"
end

The if statements will only execute if the player is wearing a shirt for the first, and pants for the second.

The next step would be to insert new shirt and pants objects if they do not have a shirt or pants, or if they do to set their shirt and pants' asset IDs to that of the clothing you wish to have the player wear.

The final script, then will look like this

game.Players.PlayerAdded:connect(function(player)
    if player:FindFirstChild("Shirt") then
        player:FindFirstChild("Shirt").ShirtTemplate = "[insert shirt asset here]"
    else
        local shirt = Instance.new("Shirt")
        shirt.ShirtTemplate = "[insert shirt asset here]"
        shirt.Parent = player
    end
    if player:FindFirstChild("Pants") then
        player:FindFirstChild("Pants").PantsTemplate = "[insert pants asset here]"
    else
        local pants = Instance.new("Pants")
        pants.PantsTemplate = "[insert pants asset here]"
        pants.Parent = player
    end
end

Hope this helps!