How do I include two "if" statements into a script?

You should just be able to use the and operator to check both in the same line (nested if statements would work too). It is also important to check if hit has something called Decal inside before checking the transparency of the decal to avoid errors.

if hit.Parent.Name == 'floormesses' and hit:FindFirstChild("Decal") and hit.Decal.Transparency > 1 then

However, since the maximum transparency is 1 (1 is completely transparent), you might want to rethink this check. If you want to check if the decal is not transparent, it would be Transparency < 1.

If I understand what you're trying to do correctly, then wouldn't it be something like this?

--drgigabyte
local broomOne = script.Parent.Parent
local messOne = game.Workspace.floormesses.mess1
local messTwo = game.Workspace.floormesses.mess2
local messThree = game.Workspace.floormesses.mess3
local cashOne = game.Players.LocalPlayer.leaderstats.Money

script.Parent.Touched:connect(function(hit)
    if hit.Parent.Name == 'floormesses' then -- the first if statement
    if hit.Decal.Transparency > 1 then -- the second if statement
            wait(.2)
            hit.Decal.Transparency = 1
            cashOne.Value = cashOne.Value + 10
            wait(.1)
            script.Disabled = true
            wait(6)
         script.Disabled = false
    end -- end to the2nd one
    end -- end to the 1st one
end)