I can't shift to the largest chainring Please help. The derailleur seems to be fine, I'm suspecting the handle. I can't shift to the largest chainring at the front, it drops back to the second largest ring as soon as I release the shifter. I need to keep the pressure on the shifter in order for the derailleur to maintain its position on the largest ring, or else it will drop back to the second one.
Let’s quickly review how your bike shifts. Your front derailleur rests in the small ring position. When you move your left shifter to the big ring position, you pull cable, and that pulls your FD to the outside position. The same principle applies to the rear derailleur, which rests on the smallest cog, and gets pulled to the big cog. There's a ratchet inside each shifter that controls the amount of cable it pulls per shift.

The thing with cables is that they stretch over time. So, there may be a bit of slack in the cable, and your shifter isn't pulling as much cable as it should. This means your front derailleur doesn't move as far outward as it should. This happens with new bikes - I distinctly remember this happening on my first bike, and I was having trouble shifting both front and rear. The same issue will probably happen to your rear derailleur.

If this is a new bike, chances are the bike store asked you to come in after a few weeks for a free tune up - they’d adjust your cables. You can take them up on it. Alternatively, you can adjust your bike yourself. It likely seems complex at first, but the mechanisms are fairly simple once you understand them. Park Tools has a nice post explaining how to do this, and the post also has an embedded YouTube video. If it’s just cable stretch, you will only need to twist the cable barrel adjuster, and you can skip the bits about setting the limit screws.

Basically, righty tighty (I.e. turn clockwise to tighten) on almost all bolts on the bike. When you turn the cable adjuster clockwise, you push the barrel further in, which slackens the cable. Turn it anti-clockwise instead to tighten the cable.

If you have gone and adjusted your limit screws, you’ll need to re-adjust them carefully. The symptoms you describe can be explained by new bike and cable stretch, so I will not describe the limit screws right now.

(A previous version of this answer wrongly described what to do for just the rear derailleur. The title of the post initially said "cog", which is standard terminology for the gears in the back. We would say "chainring" to mean the rings on the crank, i.e. the front. The principle is the same, it's just that cable stretch affects each derailleur in a different direction.)