How to random generate objects on the X axis?

I am busy with a project of my own and I am having trouble at a certain part. I want to have a random that generates my pictureboxes on a random X axis location. I'll send you a pic of the looks of my application and below that, some code.

I am very new to programming so "high class" and very smart solutions are still unknown for me. If awnsering this question. Be kind please :)

As you can see. All the Black bars are alligned to eachother and the Y and foremost the X axis stays the same. So the game is predictable.. it only has one opening that the black picturebox on the bottom need to go trough..

Here is the method that I made for the position of my "enemys/black bars" Btw I added a field on top that has "int position on 0"

    public int VijandPositie()      // methode om de positie van de vijand te bepalen
    {
        positie += 1;
        return positie;
    }

I implemented this method into this timer_tick event.

    private void timer1_Tick(object sender, EventArgs e)        // timer voor het te laten bewegen van de vijanden
    {
        Vijand1.Top = VijandPositie();
        Vijand2.Top = VijandPositie();
        Vijand3.Top = VijandPositie();
        Vijand4.Top = VijandPositie();
        Vijand5.Top = VijandPositie();
        Vijand6.Top = VijandPositie();
        Vijand7.Top = VijandPositie();
        Vijand8.Top = VijandPositie();

        if (Vijand1.Top >= gbSpeelVeld.Height || Vijand2.Top >= gbSpeelVeld.Height || Vijand3.Top >= gbSpeelVeld.Height || Vijand4.Top >= gbSpeelVeld.Height || Vijand5.Top >= gbSpeelVeld.Height || Vijand6.Top >= gbSpeelVeld.Height || Vijand7.Top >= gbSpeelVeld.Height || Vijand8.Top >= gbSpeelVeld.Height)
        {
            positie = 0;        // zorgt ervoor dat de positie gereset wordt op 0 en dan begint de pijl weer van boven af aan
            score = Convert.ToInt32(lblScore.Text); // score wordt geconverteerd naar de label
            score = score + 1;
            lblScore.Text = score.ToString();
        }

after the black bars reach the end of the screen they will get back on top again but the X axis location stays the same. I know that I have not putted a random inside yet.. so I know that this will not work. But I don't know where or how to implement the random to get the blackbars randomly on the X axis (Y axis is less important IMO).

Thanks in advance guys!