Concrete leaches lime into the soil around it raising the pH. Which might be a good thing depending on where you live. You should know your pH of your soil, the ideal pH of each type of plant you plan to grow and how to manipulate it up or down in tiny increments.
I was surprised no one mentioned 'TREKS'...I think that is the correct spelling...it is made out of recycled plastic jugs, never rots. Comes in lots of colors but I always use dove gray as that is the natural color of exposed wood. Great for edging gravel walkways and decks, too. Easy to bend when warm (use a propane torch carefully if the weather is cool). It isn't as cheap as wood or cinderblock but you won't have to line it. Saw it like wood. You will need a drill bit to drill holes for your screws.
And you really don't need sides like one of your earlier answers. I double dig my beds once at the beginning and never do it again. I always have a trench at the bottom of my beds to direct extra water away from walkways and the beds to prevent erosion. I do retrench once a year (looks so nice) throwing the soil on top of my beds, use a green cover crop every winter and topdress after planting with a decomposed organic mulch. The soil organisms eat the mulch, go back down into the soil and poop it out doing the mixing and aerating for you!
After double digging your beds, you'll see at least 2-4' of bed height. I make my beds 3' wide with walkways on at least one side. Rake the top so that it is level then I use a big piece of plywood, lay it on top, get up on it and jump up and down to firm the soil to a proper density. By now your beds will be 1 1/2' to 2' in height. They will reduce a little more depending on the amount of organic matter that originally came with your topsoil and/or the decomposed organic matter you mix in while double digging.
I never waste an inch of my beds...no straight single rows for me. I broadcast my salad bowl seeds, after soaking peas and beans I arrange them all over the top about 3" apart and push them lightly into the soil and I stagger my corn, tomatoes, potatoes making triangles with equal sides. For the seeds I'll use a rake and standing on one side of my bed to reach the opposite slope, I lightly rake up the sides and flip soil on top of my seeds. You'll have plenty of practice while forming your beds and develop confidence you'll not bury your seeds too deeply. Just pay attention to the directions on the seed packet or twice the thickness of your seed should be the depth...shoot, I think that ratio is correct.
Then I take my rock rake the heavier one with study metal tines, turn it up on its head and tamp down the soil over my seeds. Seeds need to have proper contact with the soil...just don't overdo the tamping thing, and maybe a bit more gentle over your soaked seeds. Careful with fertilizer until you get sturdy plants. And don't forget the sides! I plant strawberries or other low-growing plants there...the north side is a good place for a second round of peas in the summer and the south side make sure the plants don't shade the rest of the bed. I also make big square beds...6'X6' (3' is the maximum comfortable distance from the walkway...) with walkways on each side for peas or corn...or squash and pumpkins.
Keep an eye out for disease and insects and critters of course. I plant densely all the time. I thin and prune to allow for as much ventilation as I can. I have fans in my greenhouse and I'll even use them outside if it is humid and windless. Remove any decaying material and remove spent plants after harvesting. Put in your compost heap.
Make sure you get your soil tested otherwise you'll be guessing at whatever you add to the soil. Then get to know the signs/symptoms in your plants of deficiency AND excess nutrients. I like to use a cheap oscillating sprinkler to water my garden, make sure the moisture gets 6" deep in your soil (hand water with a soaker while seedlings...have to keep the top of the soil with your seeds moist, not wet until they are established...then you train them by watering deeply and allowing to dry out before watering again).
Don't forget to put your beds away for the winter by growing a green cover crop. In early spring turn those plants into the soil. It feeds the soil and greatly reduces weeds!
Oh, the salad bowl thing...I take all my seeds of lettuce, spinach, radishes, carrots mix them in a spice jar with holes on the top and cover the top of the salad bowl bed...thinly. You'll love this...The first to come up are radishes, you eat those and it leaves more room for the lettuces which as you eat those you are thinning. When you get through the season and there's plenty of room for all the plants just cut leaves off your lettuces instead of pulling the whole plant. Michael Dirr taught me how to do this...He is my garden guru, grin!