Once you enter France by a ‘long sejour’ visa, your visa should be validated by OFII (French Office for Immigration and Integration), in the first 3 months of your stay.

You need to go to this link, then enter your passport number, and follow the instructions! its so easy and fast.

Then, you may download the attestation as proof of visa validation until the expiration of your current visa.

more info: http://franceetcroissant.com/validation-of-your-visa/