landscaping on a slope

I'm new to lanscaping/gardening and have a new property which is on an intown location that sits on a hill in NH. The property slopes down to both the east and to the north, which is the backyard area.
The slope is walkable. The soil is moist well drained to well drained and loose with gravel and stone.
What is the least expensive way to build retaining walls? We'll be doing it ourselves, so is there any labor saving techniques?

The cheapest retaining walls to build are ones made of pressure treated wood rather than stone or cement--stick some 4x4's into the ground and cement them in, then attach 2x12's to them. Or you could use landscape timbers instead of the 2x12's. If this is DIY project, I would make sure the retaining walls aren't too tall--out here I think if they're 3' or more then you need permits to build them and it's really better to get the pros to do it, but if yours are only going to be a foot or two tall then it's more of a DIY project. The other important thing no matter what you build the wall with is to make sure you allow for drainage from the soil that's being held back, otherwise pressure can build up behind it and cause the wall to blow out. My landscapers installed a full drainage system behind my retaining walls, you can also just put weep holes here and there to allow water to drain

If the slope is walkable though you may not need retaining walls, are you doing this because you want flatter areas to garden, or are you doing it because you think you have to have walls because it's a hill? I have two hills, one in the front yard and one in the back, the front is not as steep as the back but still steeper than yours (you can walk down it, but you need to be really careful not to fall!) and the back one is steep enough that it's pretty dangerous to try and climb down it. Both of those hills were holding their own with no retaining walls, in the back I decided to have some walls put in so that I could garden there easily (it was covered with a groundcover of ice plant when I moved in and I wanted something more interesting), but in the front I left the hill alone and planted it, then had steps built to make it easier to get down.

Thanks. Your info is helpful. I do want flatter areas for a patio and a vegetable garden, so those are probably the only two, and they would not exceed 3'. The steepest area is a 10'X40' that borders a busy 2-lane route. That area is steep, although grass is growing on it, I have concerns about mowing it myself and would like to naturalize it with native low growing shrubs if possible.

If you want to see pics of my hills, I have before and after landscaping pics in my diary so you can see the hill that I left alone, as well as the one that I had the retaining walls done on.

I really liked your steps and dry creek bed. What kind of lumber and stone did you use for the steps? That's what I had in mind for my side yard

The steps are made of 4x4 pressure treated lumber, and the stone is very fine gravel that's been tamped down to make a firm surface. I had a landscaping company do the design and installation so that's about all I know--unfortunately won't have any good tips for you on how to build something like that! But I can offer to take more detailed close-up pictures of the steps and send them to you if that would be helpful.