Younes, if you want to add the JS code on your site, you may add it here:
www.site.com/admin/settings/page in the Custom head code section.
If you want to customise your Join page, this is considered as a custom code modification and you will need to edit the join_index.html file.
However, as you might know, we recommend not to modify the core files because when you update your site - all the changes you've made will be lost.
If you are not familiar with the code modification, it would be better for you to find a developer who can help you. You can do it here:
http://www.oxwall.org/market/specialists
Or if I misunderstood you please, rephrase your question and give more details.