Answer: Before you can plant anything you will need to remove the existing thorny weeds, either by digging them out by the roots or by using an herbicide containing glyphosate. Be sure to read the thoroughly follow the directions on the label for best results.
To prepare for planting grass seed you would need to rake away surface rocks and debris, then loosen the soil down about six inches (by spade or using a tiller) and remove any additional large rocks or stones, and then work in a generous amount of organic matter such as compost (again by spade or tiller). You would also need to test the soil to check fertility and pH. Then you would add fertilizer and/or lime based on the test results. Once you plant the grass seed you will have to water the lawn area to keep it moist until the seeds grow.
Grass requires a reasonably good soil along with full sun or a minimum of half a day of direct sun to grow and stay healthy. From your description, I am not certain the area will be sunny enough to grow lawn there. If it is partly shady to shady, you might want to consider a ground cover instead, possibly Vinca minor or ajuga or Hedera helix. You will still need to prepare the soil for planting, but the overall maintenance would be much less once established.
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