The problem is that your replacement crankset only has 46 teeth. You will seriously affect your gearing;it's kind of wimpy on this bike anyhow (read prior posts,many of us have upgraded the drivetrain). You'll seriously drop your top speed down with a 46 tooth crankset. Ride it with the stock crank for a bit is my advice. Then figure out how you want to shift the gearing.

Removing a crank requires a special tool; putting one on really requires proper torque,or it won't stay on. This is either done by feel or with a torque wrench. Feel requires experience. In addition, replacing a crank usually requires replacing a bottom bracket as well (another tool); most cranks have an optimum bottom bracket spindle width, and there's no guarantee your new crank takes the same spindle width as the old. Then you might create new issues with chain length as well.

The ninja build is a fine idea, but if you are not familiar with bike mechanics, slow down. Get the bike assembled in stock form and ride it.Then start switching parts. Learn how to change a tire and fix a flat first. You need this skill. Then move to something like changing pedals or the seat and post,they're relatively simple.

There's got to be bike co-op in NYC, find it and see if they offer classes. Much of learning to wrench a bike involves visual instruction,and although we can explain some stuff here, it's really a good idea to watch skilled hands doing bike work to learn it.

Or just get a shop to do it, but that requires extra cash. Of course,so does buying tools, but then you know how to fix a bike if something goes wrong.