Correction I just measured site, its 20 x 30'.

it depends, are you going to till and row it up or are you going to do a raised bed?

for raised beds, you don't need to do anything.

for row/till methods whatever works for you. you cna mulch it now, then pull it off and till it in spring. or you cna just till it a few times come spring. just keep in mind that every time you till you bring new weed seeds to the surface. most of the folks here who have picture perfect gardens will tell you that you MUST till it several times at least a week apart. many actually till it and row it up in the fall, let it sit all winter, then come spring mulch it BEFORE the weeds start growing and then plant. you want to leave the soil as undisturbed as possible to cut down on fresh weeds.

if your soil requires a lot of amendments, youmay want to add some in now, then do it again in the spring.

i don't mulch my paths, though many do. my FIL had best results using black plastic on his rows. he had very few weeds every time he used it. what he did was build his row, lay out a soaker hose on top of it, then cover with BP. i use the same general method last year, but i mulched with newspaper. i had a lot fewer weeds last year than i did this year with no mulch/cover.

Your plan sounds fine. Rototill the first time in the fall. Repeat in the spring.

I just did this, only a small 4 x 8 foot garden, but I did it by hand. I turned over the earth in the fall, repeat in spring while pulling dead weeds out of the clumps.

I would, for whatever that's worth, turn it over and till sometime later.

Lasagna beds is the way to go....no rototill, no weeds come to the surface....and everything grows like gang busters.
To put it simply, you lay newspaper over the grass, pile on organic matter...both greens and browns, water water, mulch and plant.
Linda C

Here is a link that might be useful: lasagna gardening

I take a knife and cut thru the grass in that area and pull it up. I'll assume the spot is in the back yard since its rather large.

As to what type of composting to get organic material, that probably depends on your experience. If you have made hot compost piles, I'd do that. I would make a number of 4' to 5' diameter piles 24" high, since that is my standard compost pile. On the spot you intend to use as a garden spot. If you have made lasagna or interbay type mounds in the past, you probably want to do that.

What materials you can use may affect your planning. I can secure 100# of used coffee grounds and 400# of veggie waste a week if I wish. Leaves are still available here since the tree shed their leaves so late this year.

But all this depends on how good your soil is in the first place.