I'm single, and I have more than the 180 something days that the presence test requires.
I think I can fill form 1040EZ because H1B holders pay taxes like a normal resident.

So my question is, can I use form 1040EZ for my taxes, or which form do I need?

What if I was working from Jan to March with my F-1 visa, and then started working with my H1-B from March - today.

Finally, is there a tax treaty for Mexicans?

Regards,
Aragorn

You MUST file as a dual-status alien.

If you are in non-resident alien status for EVEN ONE DAY, you CANNOT file as a resident alien if you are single.

Since you have met the Substantial Presence Test, you CANNOT file as a non-resident alien.

Yes, there IS a tax treaty with Mexico. See IRS Pub 901.