For the first time gardener it seems like a lot of work and skill will be required to develop a good and bountiful harvest. What I will do for you is to break it down into a short list of steps to follow to get you on the go.
There are a few steps to first put some thought into before you start to buy your plants. Fist of which will be, is the garden for fruits and veggies or is it for flowers. This is important because the amount of sunlight can have a dramatic effect on your outcome. If at all possible try to place your garden in an area that allows you to view it. Your ability to see it even when you are not working on it will help you stay motivated and it will also help you see the fruits of your labor (so to speak). Although you can plant in a completely shaded area you will be very limited on your plant choices. For most vegetables and flower beds at least 6 hours of sunlight should be given to them for best results.
One other thing to consider when placing your garden is most plants will turn to face the sun so if you are looking from a west facing window, your flowers will be facing away for you in the afternoon. I know this is not a luxury that everyone can afford but if you do have the option why not take advantage of it.
Once you have figured out where you want your garden the next step would be to lay it out on the ground. You could use a hose or a rope or even a can of marking paint from the hardware store. Whatever method you use take into account the amount of foot and animal traffic you get in the area and take that into consideration.
Next step is soil prep. I know this is not the fun part of the job but it is the most important part to have a beautiful Organic Garden. The first part to look at is the ph of the soil. Somewhere between 6.2 and 6.7/6.8 is an acceptable range for the bulk of the plants and flowers for the average garden. If you are not sure, the ph is usually found on the plant tag. To find out the ph of your soil you can usually find a nursery in your area that will test your soil for a small fee. You should also perform this test before you put lime on your lawn in the spring. Too much acidity in the soil can contribute to your lawn dyeing.
Now what to do with the existing lawn/weeds that are currently where you want your garden, this decision will dictate when you should decide to start on your new garden. If you remove the grass use a spade shovel to cut the ground and pull up the sod bit by bit. If you decide to till in the existing sod make sure to do it in the fall so the organic matter has a chance to start to decompose during the winter. Most likely you will see some grass in the spring and summer start to sprout up due to this method so I would recommend either removing the sod completely or smothering it.
Removing all the sod and hauling it away is a lot of backbreaking work so if your soil is already in good shape as far as the ph is concerned you can always take the easy route and build on top of it. First lay out eight to ten layers of newspaper on the ground where you want your garden. Next wet it all down and turn it into a thick pile of wet newspaper. Once the paper is good and wet, cover the paper with six inches of good quality soil. Over time the newspaper will break down and the grass will have become sufficiently smothered.
The next step is to decide whether or not to use mulch. There are numerous types of mulch to choose from. There is organic mulch like chipped bark, shredded leaves or straw. These mulch types give multiple benefits to your soil like good water retention, helps discourage weed growth, and keeps the soil cool. There are also examples of non organic mulches like plastic sheets that help keep the soil warm for warm season crops. Whatever your mulch choice is make sure to lay it right after planting and keep some distance from all plant stems to avoid rotting them. This also helps deter moles and mice.