How do I make it so when people join my game, they wear a certain shirt?

This will work.

local shirtid= --Put the ID here. No need to subtract one, the script does that
local pantsid= --Put the ID here. No need to subtract one, the script does that

Game.Players.PlayerAdded:connect(function(p) p.CharacterAdded:connect(function() 
if p.Character:FindFirstChild("Shirt") then
p.Character.Shirt:Destroy()
end
if p.Character:FindFirstChild("Pants") then
p.Character.Pants:Destroy()
end

local shirt=Instance.new("Shirt")
shirt.ShirtTemplate="http://www.roblox.com/asset/?id="..(shirtid-1)
shirt.Parent=p.Character

local shirt=Instance.new("Pants")
pants.PantsTemplate="http://www.roblox.com/asset/?id="..(pantsid-1)
pants.Parent=p.Character

end) end)

If I were you, I would remove their current one and either clone one in from lighting or insert a new one with a certain AssetID.

game.Players.PlayerAdded:connect(function(plr)
    s = Player:FindFirstChild("Shirt")
    if s then
        s:Destroy()
    end
    Instance.new("Shirt").AssetID = IDhere
end)

That should do it.

Change their character's shirt's ShirtTemplate property to the asset id of the desired shirt template.

Same with me but i want to know how to do it with the whole body instead of just one part