I have overgrown brush and wild plants. What's the best way and easiest to get rid of them and plant grass?

Any of the brush (plants, weeds, bushes, etc.) should be sprayed first. You can purchase pre mixed Roundup at any Home Depot, Lowe's, Orchard Supply, Tractor Supply as well as a good backpack sprayer. You can also buy the concentrate and dilute yourself. Once they die they are easier to mow or remove. You will also be able to notice any spots you missed since they will still be green.

After removing the brush then tackle the larger bushes and trees. With the little stuff out of the way it will be easier to get to the big stuff. Depending on the size of the area and how much help you have you could be done in only a couple weekends if not sooner.

In response to the additional answer, you would not have to worry at all about spraying as Roundup is a foliar spray which means it is ONLY absorbed through a plants' leaves. If you spray, and clean up the area there is NO possiblity of chemicals affecting produce or grass. Also something to note, table salt has a higher toxicity than Roundup.

Believe it or not, people rent out herds of goats to trim large areas. They work cheap, can scramble up steep hills and eat every sprig in their path.

If you can't do that I would start by cutting small areas with a weed wacker or some kind of trimmer. Then come back to tackle the larger shrubs. You may decide you want to keep some of the shrubs as a transition to the woods beyond.

Either way I don't recommend spraying. If you decide to plant vegetables in that area you will have to worry about the chemicals you used. Good luck.