Younes, the simplest thing if I understand your question is to:

Login to your Admin Panel

Click on the "Pages & Menus" Icon at the top

You should see a Main Menu that might look something like this:

Now Drag & Drop the Join Tab to the far left so it looks like this:

Now since the tab at the far left is the default Index... when a user just lands at your site they should land on the Join up page...

thank you very much :) 

it is working now :)