3 week old brand new Montra Rock 1.0 front gears don't change as well chain slips frequently
Ensure the shifter is set to the low position - the position where the chain is on the smallest chain ring.  Give the cable a tug, it should be taut but not tight. You can make minor tension adjustments by turning the barrel adjuster near the front shifter-to-cable housing connection.  If you can not take up enough slack with the barrel adjuster, screw it in about 3/4 the way and then loosen the cable at the front dérailleur and pull out the slack in the cable and retighten. Make the final adjustments with the barrel adjuster near the shifter. It should take about 2 minutes for correct.

If you continue to have shifting problems it can be the angle of the derailleur or the position of the set (stop) screws. In that case if you are comfortable making adjustments look to YouTube for instructions, otherwise, take it into a shop.