if DIY, concrete block walls, or other form of block is way to go. that or Tie walls. tie = 6" x 4".
poured concrete walls = more likely better off contracting work out. or less you can count on some good friends with muscles to back them.
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personal preference is going with tie walls. by the time they need to be replaced 25 plus years from now. the wood will have rotted a good amount and the weight is much less to a point were it is much easier to handle pieces. from the old wall.
tie walls, granted you need to put in your dead man. dead man = tie that runs perpendicular to the wall. or rather runs back into the dirt. it can help keep walls from falling over.
tie walls for myself tend to be easier putting up. small chain saw, 2 to 3 lbs hammer, and Spikes (large nails). and 2 drill bits (1/2" or or so, depends on spikes) to pre-drill through top tie, shovels. and a tractor makes things easier moving dirt and stuff. and a good 4 foot level and a smaller level.
getting that initial starter row, and a little re-work for second row. is a must. and what takes the longest. after that. for most part it is just pulling the ties down to were they need to go. make a cut if need be, drill through top tie, place spike in, and hammer it down and move on. exception for the dead man, they take a little extra work having to dig back into the hill. if you have a backhoe with a small bucket on it. that makes things quick work.
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for concrete block walls, the footer can be a pain. getting rebar and poured concrete in. and then getting all the blocks nice and level on the first row.
for a novice i might say, get your footer poured first and get it nice and level. and then come back and just place blocks down for first row to get them were you want them. then remove blocks as needed. and use a masonry bit (concrete bit) with a hammer drill to put your rebar in. it is not perfect but. trying to align vertical rebar in footer before pouring the concrete for footer and getting them just right for holes in concrete block can be a pain in the rear.
concrete mixer is almost required, or less you are in really good shape. personally prefer a 2 bagger (concrete mixer able to handle 2 80lbs bags) prefer a mixer that stands up off the ground and dumps into a wheel barrow.
any bigger and not worth it, at least to me, or less it was my day job. any smaller and it makes it a pain to deal with. or less putting a little concrete edger around a flower bed.
pending on length and height of walls. you may need additional horizontal rebar every so many courses. concrete block walls can get complicated. but they are fairly easy to put together. once ya figure out were to place rebar.
another thing is making a "small wooden shout" for dumping concrete from wheel barrow into the concrete block holes. just some plywood or better yet 1x6 or like lumber. to form a mini slide. (like a kids swing set slide but shorter in length) or you might think of it as a funnel, that just fits over top of the block. and gives you a wider area to dump concrete from wheel barrow into the block holes.
if you never mixed concrete before. getting the right consistancy of concrete mix and water (most likely ready mix and just add water) takes awhile but once figured out. it pretty easy going.
