My first day on this website

We use on for websites and the internet. In is suitable for groups and communities.

I found out about this wonderful website from a post on Facebook twelve hours ago.
Since then, I've spent all my time here because I'm enjoying the fact that there are many languages learners here, and the participating is so easy.

Use find out for discovering information or facts. Since usually goes with present perfect (I've spent).

I'd like to thank a lot for those who created this great way to help us improve another our second languages.

These corrections just make the message clearer. Be careful of casual forms - we don't use them with people we don't know, so a more polite form (I'd like to thank...) sounds right.

I'm looking for good friends to practice English together, and be like native speakers.

Who would be like native speakers? Yourself, or your language partners?

I'll help anyone who wants to learn Arabic and I appreciate your corrections of my mistakes. :)

Again, using a slangy casual form ("wanna") has caused you to make a mistake... a double-mistake, in fact. Just use standard English and you'll have no problems. That's exactly what we native speakers do. ;)