Anyone know how to script fireball projectiles?

This should be pretty simple

Though I won't write code for you these are some links you're probably looking for.

Helpful Links: https://developer.roblox.com/en-us/api-reference/class/Tween https://developer.roblox.com/en-us/api-reference/datatype/CFrame (You could also use Vector3) https://developer.roblox.com/en-us/api-reference/property/Mouse/Target

Using these 3 tools could just about make a fireball, and using your code for a debounce and Imput service detection you should be good to go to use these for a function that creates a fireball.

Try putting the fireball model in replicated storage and using a remote event to clone it and then tween it towards the mouses target.

Localscript, you can put it inside PlayerGui, StarterPack, StarterPlayerScripts, StarterCharacterScripts

local player = game.Players.LocalPlayer -- getting player
local uis = game:GetService("UserInputService") -- getting UserInputService
local db = true

uis.InputBegan:Connect(function(input, playerIsChatting) 
    if input.KeyCode == Enum.KeyCode.E then -- checking is input that player did was letter E
        if not playerIsChatting then -- checking if player wasnt typing 
            if db then -- checking if db(debounce) is true. If false then it cant go through and stops here
                db = false -- db is now false
                workspace.FireballEvent:FireServer() -- firing remoteevent inside rep storage (we need remote event so other players in game can see the fireball)

                wait(5)
                db = true -- after 5 seconds db is true again and player can shoot fireball again

            end
        end
    end
end)

now put RemoteEvent inside Workspace and Server side script (normal script) inside RemoteEvent

Server script

script.Parent.OnServerEvent:Connect(function(player) 
    local fireBall = game.ReplicatedStorage.Ball:Clone() -- making path to Fireball inside rep storage and also cloning it

    -- make sure fireball isnt anchored

    local char = player.Character or player.CharacterAdded:Wait() -- waiting for players Character
    local HRP = char:WaitForChild("HumanoidRootPart") -- waiting for HumanoidRootPart. good thing is that HumanoidRootPart has r6 and r15 avatars

    fireBall.Parent = workspace -- new parent is now workspace
    fireBall.Position = HRP.Position -- fireball position is now at center of HumanoidRootPart

    local bodyV = Instance.new("BodyVelocity", fireBall) 
    bodyV.MaxForce = Vector3.new(math.huge, math.huge, math.huge) -- setting max force to vector3 math.huge... (math.huge is infinity so it wont fall down)

    bodyV.Velocity = HRP.CFrame.LookVector * 120 -- direction where fireball will go, HRP.CFrame.LookVector is where the hrp is looking (Front Surface)

    local hitDb = true
    fireBall.Touched:Connect(function(hit)

        if hit:IsA("BasePart")then -- if fireball doesnt hit accessory or something like that
            if not hit:IsDescendantOf(char) then -- if hit isnt part inside character so we cant damage us

                if hitDb then -- if db is true than it can go if false then it cant do damage
                    hitDb = false

                if hit.Parent:FindFirstChild("Humanoid") then -- hit can be Arm or Head so we want Parent of that part so that will be players Character, now we are checking if hit.Parent have Humanoid

                        local Ehum = hit.Parent.Humanoid 
                        fireBall:Destroy() -- if hit player it will destroy fireball
                            Ehum:TakeDamage(20) -- if yes, we can deal him damage
                    end
                end
            end
        end
    end)

    wait(5)

    fireBall:Destroy() -- fireball will destroy after 5 seconds
    hitDb = true -- hitDb is true so fireball can damage again
end)