I am in usa from August with wife and one kid. Wife is on H4. Can I file joint return? Can I calim son as exemption?

Please advise.

You can file resident tax return as Married Filing Jointly.
You will have to wait for 4 to 5 months to file your tax return till you meet the Substantial Presence Test in 2009.

You will get standard deduction of $10,900 and exemptions for both of you and the kid. You will also get child tax credit of $1,000 each.

You must both declare your worldwide income for 2008. If you have any foreign income, and on that income you paid taxes in the foreign country, then you can claim foreign tax credit (Form 1116) or you can use Foreign Earned Income Exclusion (Form 2555).
Read about H1-B tax filing: Your U.S. Tax Return: The U.S. Visas (http://taxipay.blogspot.com/2008/08/us-visas.html)