Depends on the kind of desert
Some deserts already have useable soil, they are just really dry places that need irrigation as the accepted answer has pointed out. However, other deserts are not just dry, but thier soil is completely ruined; so, these guys need a lot more work.
If you're dealing with a desert where the landscape looks cracked, that means that the soil already has clay in it, and probably already has enough bioavailable resources to get started. However, if you are talking about a sand desert more like the Sahara, then just adding water is not enough. You'll need to do some soil restoration first before the land becomes agriculturally sustainable.
There are places in North Africa where sandy deserts are used for agriculture, but because the soil was not repaired first, they require about 4 times as much water and fertilizer to maintain as other farmlands, and unless the farmers are very careful with thier fertilizer blend, it's easy to make the soil toxic over time with neutrants that retain in sand better than others.
So if you are dealing with a sandy desert, you will need to do the following:
Step 1: Improve water/nutrient retention
The hardest part about farming a sandy desert is not the heat, the dryness, or even the salt. It's the inability for the sand to retain water and nutrients. When you place fertilizer in pure sand, and then add water, the water causes most of the components of the fertilizer to wash away leaving you with inert sand which most plants can not get enough micronutrients out of to survive.
The best way to solve this is to import clay which you can mix into the sand to create lome. When water mixes with clay it makes the clay sticky and bind to the sand allowing water and nutrients to be retained better.
Since this is a desert, you will probably also want to add charcoal to the soil. It will further increase the soil's water retention and add small amounts of certain biologically available nutrients to the soil to help get things kick started.
Cereal grasses especially tend to prefer loamy or clay based soils because they are especially good at breaking up harder soil which helps them outcompete many kinds of weeds; however, if you do not have the resources to import tons of clay and charcoal, it's better to pick a crop that prefers more sandy conditions anyway. I'd suggest tomatoes, peppers, garlic, onion, or cucumbers for a hotter desert, or carrots and potatoes for a colder desert.
Step 2: Fix soil acidity
Most deserts tend to be far too alkaline for most crops, and most agricultural plants make the soil even more alkaline. Many deserts today exist because of bad farming practices depleting soil acidity; so, you need to plant starter crops that will help with this. Starting off with trees is generally a good first step because they will make the soil more acidic, uptake the extra salt, and their large root structures and high foliage are especially good are preventing erosion as the soil builds. Evergreens are a popular choice because they grow quickly and can be harvested in as little as 10 years for thier lumber.
If you don't want to wait, you could instead try adding sulfur to soil. This is only a so-so solution because it still does not address the extra salt or other scarce micro neutrants, but it does get the job done. It will acidify the soil making it good enough to start growing stuff, though your soil will be far less stable than if you have those wide deep tree roots left in the ground holding everything together
Again, it's typically better to plant what grows well in your given soil than just planting what you want. A better solution for overly acidic and salty soil could be olive trees. They take a while to mature compared to other crops, but they are very long lived, and thier oil is a particularly profitable cash crop. They are also a good companion crop because they are so good for soil conditions; so, you might want to plant rows of olive trees with the plan to grow other stuff in between.
Step 3: Keeping the soil productive
One of the big problems with agricultural desertification is a poor choice in cover crops. It's common knowledge that wheat takes up nitrogen and legumes put it back, and some other thing can be planted in winter to prevent erosion, but soil is more complex than this. If you are already coming from the brink of failed soil, you want to make sure your crop choices leave your land better each year you plant. Not worse.
For this you will need a mix of cover plants that can scavenge micronutrients from that sand, build biomass, aerate the soil, encourage microbial growth, attract pollinators, etc. One mix that's been gaining popularity lately is the 5-Cousins method: Amaranth, Cowpeas, Buckwheat, Daikon Radish, and Sorghum. Sown together, these guys will build up your desert soil year after year into progressively better soil needing little to no outside fertilizers and a minimum of water.
