Part Generation

local CurrentPos = Vector3.new(0, 0, 0)

for i = 1, 100 do 
    wait(0.1)
    local Part = Instance.new("Part", Workspace)
    Part.Anchored = true
    Part.Size = Vector3.new(2, 2, 2)
    CurrentPos = CurrentPos + Vector3.new(2, 0, 0)
    Part.Position = CurrentPos 
    if i % 10 == 0 then
        CurrentPos = Vector3.new(0, 0, CurrentPos.Z+2)
    end
end

The above would do as you require.

The modulus operator, %, is used to return the remainder of the division between two given numbers. In this case I'm using it in an if statement to check if something is a multiple of ten, when it is it starts a new line on the Z-axis (allowing for a 10x10 square).

The rest is pretty self explanatory.

for x=0,9 do
    for y=0,9 do
        local e=Instance.new("Part")
        e.FormFactor="Symmetric"
        e.Size=Vector3.new(2,2,2)
        e.Position=Vector3.new(x,0,y)*2
        e.Parent=workspace
    end
end

or if you only want one for loop:

for i=0,99 do
    local e=Instance.new("Part")
    e.FormFactor="Symmetric"
    e.Size=Vector3.new(2,2,2)
    e.Position=Vector3.new(i % 10,0,math.floor(i/10))*2
end