Many people make a really big thing about spraying poisons to kill "weeds" before tilling and then people find that seeds germinate and grow even after spraying.
Tilling, without spraying, will put that unwanted plant growth, the "weeds", into the soil as organic matter, something that soil may well need.
I would start with a good reliable soil test for soil pH and major nutrient levels and even use these simple soil tests,

1. Soil test for organic matter. From that soil sample put enough of the rest to make a 4 inch level in a clear 1 quart jar, with a tight fitting lid. Fill that jar with water and replace the lid, tightly. Shake the jar vigorously and then let it stand for 24 hours. Your soil will settle out according to soil particle size and weight. For example, a good loam will have about 1-3/4 inch (about 45%) of sand on the bottom. about 1 inch (about 25%) of silt next, about 1 inch (25%) of clay above that, and about 1/4 inch (about 5%) of organic matter on the top.

2) Drainage. Dig a hole 1 foot square and 1 foot deep and fill that with water. After that water drains away refill the hole with more water and time how long it takes that to drain away. Anything less than 2 hours and your soil drainsâ too quickly and needs more organic matter to slow that drainage down. Anything over 6 hours and the soil drains too slowly and needs lots of organic matter to speed it up.

3) Tilth. Take a handful of your slightly damp soil and squeeze it tightly. When the pressure is released the soil should hold together in that clump, but when poked with a finger that clump should fall apart.

4) Smell. What does your soil smell like? A pleasant, rich earthy odor? Putrid, offensive, repugnant odor? The more organic matter in your soil the more active the soil bacteria will be and the nicer your soil will smell.

5) Life. How many earthworms per shovel full were there? 5 or more indicates a pretty healthy soil. Fewer than 5, according to the Natural Resources Conservation Service, indicates a soil that is not healthy.
to learn more about the soil I had and plan on what needs be done to make that soil into a good healthy soil.

Then I would till that soil, including any "weeds", and grade it, install any irrigation and other hardscape and then seed or sod the lawn.
You could spray and in a couple of weeks lay some sod and have an instant lawn and then spend a lot of time, energy, and money on upkeep.

There is a CA forum. The locals would have the most appropriate and effective advice.

The mixed advice about tilling is likely from the fact that many plants can be multiplied by tilling, not killed. Some plants only need a small piece of root to keep growing. When tilling a plant like that, chopping it up makes hundreds of baby plants. If plants are definitely dead, tilling them into the soil once, to get started, could be a good plan.

When the ground is disturbed, the layers and drainage are ruined, and much of the microscopic life is killed. A spot that has been mowed for years is probably virtually lifeless anyway though, and may be rock-hard, another instance where tilling initially could be helpful. The info here explains this well.

Time to start learning about compost, and using unwanted plants, and other organic matter to your gardening advantage. There is a "soil/compost/mulch" forum as well.