My husband is on h1b since Feb' 2012
I was working in India and visited US once in 2012 for 4 weeks and then joined my husband in Jan 2013 eventually.

Should he be filing tax as 'Married filling jointly' or 'Married filling separately'?
Do I need to show my Indian income if we are filling it as 'Married filled jointly'?
Since I was in US for less than 31 days in 2012, does that mean I will be considered as 'Non Resident Alien'?
Also, since I joined him in 2013, does that mean we can't file taxes as 'Married filling jointly'?

Please advice.
Thanks in advance

You can file jointly with your husband irrespective of your physical location; you could be on Mars for all the IRS cares, just as long as you sign the tax return.

It is probably most advantageous for you to file jointly with your husband, filing Form 1040 with Form 1116 to claim the Foreign Tax Credit.  Form 1116 is required because you must claim ALL world-wide income earned in 2012, and the Form 1116 allows you to claim a credit for the income taxes paid to your home country.

You will need to apply for an Individual Tax Identification Numbers (ITIN) for yourself by submitting Form W-7, and that process has undergone some radical procedural changes this past year.  Instead of the simple notarization process, the IRS now requires that your home country certify the photocopy of your passport as a true copy.  This is due to rampant fraud in the ITIN submission process.  The notaries simply cannot tell the difference between a fake passport and the genuine passport.

This certification can be done by the Indian (I assume you are from India) embassy in Washington, D.C. or at one of the consulates in San Francisco, Chicago, Houston or New York.  Assuming you do NOT want to drive there, you need to call the nearest consulate and arrange to have this process done via the mail.  Since you must wait until June to file, you have time to get this done (the process, done via mail, can take up to three months to complete). My clients to date have had the best results by mail with the San Francisco consulate, getting the certified passport photocopies in less than 3 weeks.

Also, the IRS recently announced the opening of Tax Assistance Centers through which you may be able to have the passport verified as genuine, then submit the tax return and Form W-7 IN PERSON to be forwarded to the ITIN Processing Center in Austin, Texas for processing , without having to get the photocopies certified by your home country consulate.

You go to the TAC with your husband, your passport, the SIGNED Forms W-7, and the completed and SIGNED tax return.

The IRS official verifies the passport, asks a few questions, completes the certification paperwork, checks the Form W-7 and tax return, then returns the passport with a receipt for the tax return and W-7, which is sent to Austin, Texas for processing.

You leave the TAC with your husband, your passport and a receipt for the Forms W-7 and the tax return.

You would get the ITIN about six weeks after you left the TAC.

The list of the TACs are at the link below:

Taxpayer Assistance Center Locations Where In-Person Document Verification is Provided

The alternative is to mail in the actual passports themselves, something most of my clients are reluctant to do.