Pro Landscaper/ lite construction guy here.
No, you are certainly not in over your head. I will chime in and say level isn’t as much a concern as straight is. in fact a slight slope from one end of the bed is disirable as it can assist in run off during heavy rain.
Best way to attain a nice straight wall is to set one stone at your height, then measure out where the last stone in a straight line will go, set it’s height where you want it, then run a string that sits on the top of both tightly between the two. Place the stones on either compacted sand, or in your case (with such a short wall) plain ol dirt will do. You can adjust the height of the individual stones in the first course by adjusting the compacted sand base the first course sits on. Just give your string a little “twang” after you place the stones to make sure the string is just barely kissing the top of the stone you just placed.
Tamp tamp tamp, make sure your first course is sitting on a solid base, step back and eyeball your line to make sure it is going where you want. Taking a 2x4x8 and smacking the face of the bricks (like an 8’ long straight edge)will help keep them straight as you progress.
In an unmortared retaining wall all subsequent courses will follow your first course so if you get the first run correct you are home free. be sure to place the first brick exactly centered on the space between the two beneath it or it may bite you on the butt when you go to finish the ends. Half bricks are good for this but if you wind up a little off, a hammer can remove some material from the brick with ease.
Drainage is easy, plain ol gravel (not too fine) will work, a perf pipe imbedded in the gravel with a positive slope will work better. Not really too big a concern with the type wall you are planning, it’s not really tall enough to worry about and the butt joints drain pretty readily on their own.
Corners are up to you, curved or square its a matter of esthetics, square joints will line up better while curved ones will “overhand” or “underhang” a bit. Nothing major.
Other concerns to address before starting are “how will this wall effect the drainage of the lot?”, Does water still have a place to go? Also whatever you do, make sure your new soil bed does not slope towards your house and that the soil is a minimum of 6" below the siding of any structure.
I Hope I covered everything here. My tool list would include a flat tipped shovel, line level, torpedo level, wheelbarrow, a couple of buckets, masons twine, a short screed stick (1"x2" stick cut slightly longer than the width of you wall block) for spreading sand, and a 2x4x8 “helper board” for lining up the front and tops of the block. You may want a good pair of gloves if you are unused to handling masonry with your bare hands.