12 x 48 is not a very big yard. You don't say if this is a front or back yard or if you have an HOA. In general, you will want to plant low water use, drought tolerant plants. A drip irrigation system on a timer is a given. Since desert plants need to avoid being eaten in their natural habitat, many have thorns, not just cacti. If you have kids and/or pets, you will need to take that into consideration. For most plants, there is no benefit in buying the 5 gallon size, the one gallon will be just fine. Most grow quickly and will overcome the size difference in a year or two at most. Not many trees are suitable for a small space and you will likely have to prune a tree regularly. The ficus is a popular small tree, but they are not cold hardy. Citrus is popular, but they require maintenance and not all varieties do well there. Cold tolerance can be an issue as well. Depending on where you are (I'm assuming somewhere in the Phoenix area), overnight temperatures will likely dip into the twenties at least a few times a year. Avoid plants that are not cold tolerant, or you will replant them every few years. Soil preparation is important, as the soil is like concrete in most places and has little to no organic material.
Lawns are a high maintenance, expensive hassle. The grass used there goes dormant in the winter and you have to overseed to keep the lawn green. HOA's love to ding you for imperfections in your lawn.
Right now, most nurseries have a low inventory of landscape plants. No one landscapes at Christmas. The landscaping crews are largely Mexican immigrants, and a lot of those folks go home for Christmas. Home Depot and Lowes are NOT the cheapest places to get the standard landscaping plants. I buy one gallon plants at a place in the Dobson Ranch area of Southwest Mesa. Been there for decades, $2-$3 less for one gallon plants than the big box stores. Inventory picks up in February to March.
When I started buying rentals, I looked at model homes for ideas. After landscaping the first new home, I changed my thinking. I started looking at street and freeway landscaping and HOA common areas. Think tough and low maintenance plants that are still attractive.
You can do a vegetable garden, but raised beds work better. Squash and melons do well. Water is very expensive, and food crops use a lot of water.
Shade is critical to being outdoors April through October. If you don't have a covered patio, you will need something that provides complete shade. Lot orientation is a factor in landscape design. North/South orientation is preferred. East/West means you will have to block the sun on the west side of the house in some way.
Any other questions, just ask.
