1. I can grab my crank arm, at verticle, and push pull it very slightly away or towards the bike and feel and hear a slight looseness..it clicks back and forth as I do this....is this bad?

2. Is replacing a sealed bottom bracket easy to do with garage tools? Is that likely what needs to happen?

There are different bottom brackets, without more information or a picture, unless someone wants to post a list of all of the possibilities (Bike model, year) more info would really help.

If Your Garage has a bunch of Specialist Bike tools in it . or you just own a Coaster Braked Steel one piece crank Cruiser ..

Looking at your bike it's likely a standard cartridge and square taper. You'd need a crank arm remover and whatever tool fits your particular BB.

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Well, that's not how it's supposed to behave. How to fix it depends on what kind of bottom bracket we're talking about. If your bike is from the 1980s or older, there's a pretty decent chance you have an adjustable bottom bracket and you just need to adjust the play out of the bottom bracket. (Might as well take it apart and re-grease while you're at it.) If your bike was made in the past 20 years, odds are that you have a sealed cartridge bottom bracket. Those aren't adjustable, so the fix is to remove & replace.

There are a couple of specialty tools you'll need, but thankfully they're generally not expensive. The exact tools you'll need will depend on what kind of bottom bracket we're talking about.

You'll need something to remove the crankset. Sometimes nothing more than a hex key (Allen wrench) is necessary. Oftentimes you'll need a crank puller like the Park Tool CWP-7 for square taper or splined cranks, once you get the retaining bolt or nut off. A decent crank puller should run you about $15.

For an old-style adjustable bottom bracket, a hook spanner and pin spanner (like the Park Tool CWP-5 & SPA-1) are the traditional tools for the adjustable cup. I usually use a big ol' 15" crescent wrench for the fixed cup on the drive side of the bike.

For a modern sealed cartridge bottom bracket, you'd need to buy the appropriate splined tool for removal and installation. For example, if you have a recent Shimano cartridge bottom bracket, you'd need something like a Park Tool BBT-32.

There's also the possibility the the crankarm is loose on the BB axle.
If the BB is loose, then when you move the crank arm in & out, the other crankarm will also move.

It turns out it must be an older style, so.'ve t has a lockring on this ne side...thanks to YouTube, I loosened the lockring, tightened the bracket a bit, then retightened the lockring....that fixed it!

IF the lock ring hadn't been loosened, you have to ask yourself HOW did the spindle get loose.
Only answer has to be wear or bearing balls going bad.
I think it would be prudent to replace the bearing balls with fresh grease.
Spend a couple $ now or more $ later.

I'm not sure...the bike is 1.5 yrs old, and cheap from Wal-Mart, so it could have been assembled badly....I feel like it's been this way for a long time, though I'm not sure how long....I've just been ignoring it.

The X mart bikes tend to have below minimal grease used.
It's probably already too late.

The first thing to do is to hold both crank arms, one in each hand, and try that same side-to-side rocking test. If pushing on one crankarm makes the other one move, you have a bad bottom bracket. If only the arm you are pushing on moves, the problem is probably your crank set.

You can do a lot of bicycle tuning and repairs with just a 5 mm allen key but cranks and bottom brackets don't happen to be one of those jobs. You'll probably need a crank puller and some bottom bracket tools and there's several different kinds. Your new bottom bracket may even require different tools to install than your old bottom bracket took to remove. Cranks and bottom brackets are one bicycle job that I do using a torque wrench. The torque spec for both parts is usually around 30 ft/lbs. that's quite a bit more than I'd use if left to my own judgement. If you don't get them tight enough, you may find your brand new left crank arm falling off and needing a replacement. Torque a square taper crank arm too much and you can make it crack at one of the corners.