I have a Shimano tourney 7 speed drivetrain.

For the life of me I cannot get the gears indexed. The bike is brand new with less than 50 miles on it.

No matter what I do the gears are not properly aligned and skip cogs.

A few pix would help diagnose.

Bunch of stuff could cause this, but a bent derailleur hanger is a good bet. Is the rear derailleur parallel to the sprockets? Many rear derailleurs take a hit at some point in their life and get bent inward at the bottom. Indexing will never work properly with alignment off.

If it does turn out to be this, don't try bending the derailleur back with your hands. There is a tool for this, the Park DAG-1 (or 2). You remove the derailleur, thread the tool into the derailleur hanger, and align it to 12, 3, 6 and 9 o'clock on the rear wheel.

I spent last evening at our local bike Co-op diagnosing several folks bikes who had rear shifting problems. All of them had scrapes on or grass/dirt embedded in their rear derailleurs. Sure enough, all of the derailleur hangers were bent. One had a pretzelized rear derailleur.

Never rest your bike on the drive side. Never drop your bike on the drive side.

If it's "brand new", have the dealer make it right.
They don't want you screwing it up and then bringing it in.

Cable length (or as most say "tension") is the first thing I look at. Andy.

Never dropped and never hit the derailleur on anything.

Will take it to the shop tomorrow I suppose. Was looking forward to riding today in the good weather /:

Tourney equipped bike. Probably a big box store buy.

Doesn't take much. Might have come that way from the store. If it's 7-speed you can probably diagnose it yourself:

1. Shift into a gear combination that makes the derailleur arm point straight down toward the floor.
2. Now prop your bike upright against something and look at the derailleur from the rear or the bike.
3. If the arm seems to be pointing inward toward your rear tire, that's it!

I own the Park tool for straightening hangers but, on a 7-speed equipped bike, I can align it adequately just by eyeball.

The lateral float, of the guide pully, makes up for minor imperfections of adjustment of the clicks..

I think the point of it is to refrain from forcing the chain to shift at the instant the derailer moves in favour of waiting for the shift point to come past the chain. Even on a 7s system, fine adjustment affects how readily upshifts happen compared to downshifts.

This video is good. Worked well for me:

Then it's probably not your fault. You'd be surprised how many new bikes have the RD hanger misaligned.

Before I started wrenching on my own, I was a shop rat at the LBS. I noticed they brought out the DAG tool on every new bike build, and almost every new bike was misaligned, even on $3000+ models. So I put that tool into my rotation of things to do on used bikes, and it's one of the first things I check.

Almost all base model bikes from Specialized, Cannondale, Giant, and Trek come standard with Shimano Tourney drivetrains.

Might help if you can better explain the problem, and how you went about trying to fix it
Who assembled it ?
Could be a kinked cable, or burr at the end of the cable housing.

Have you followed any tutorials ??

Shimano Tourney RD's are junk. The only shifters they halfway work with are grip-shifters. Toss the Tourney & upgrade to a Deore. You can find long cage Deore on ebay for as little as $30. They work like a charm.
Shimano Deore RD M591 9 Speed Rear Derailleur Long Cage Mountain Bike