How do i make all players Walkspeed 35?

Default WalkSpeed is 16. If you want to change the default WalkSpeed to 35 then change it whenever a character spawns:

game.Players.PlayerAdded:connect(function(player)
    player.CharacterAdded:connect(function(character)
        character:WaitForChild("Humanoid").WalkSpeed=35
    end)
end)

To change player walkspeed, we need to find player's Humanoid. The first thing we need to do is find LocalPlayer, then find his character in Workspace, and then find Humanoid. To find player's model in workspace, we can use Character. And finally, to find Humanoid, we can use :FindFirstChild(), and after finding humanoid, change his walkspeed.

Notice that it must be LocalScript putted either into StarterGUI or into StarterPlayer > StarterPlayerScripts

Here is the code:

Player = game:GetService("Players").LocalPlayer --Finding our player

Character = Player.Character --Finding player's model in Workspace

Humanoid = Character:FindFirstChild("Humanoid") --Finding Humanoid

Humanoid.WalkSpeed = 35 --Changing Humanoid speed

We can also write it in single line to make it much smaller:

game:GetService("Players").LocalPlayer.Character:FindFirstChild("Humanoid").WalkSpeed = 35