to kill a garden what is the best product to use?

I think you have two options for removing perennial plants to make way for grass.

Roundup is one product. It will require several applications on the harder to control plants like the ivy. But will work. Keep retreating as new growth appears. You will be able to plant the grass in early September.

Broadleaf herbicides such as 2,4-D or triclopyr are more effective but still might need a follow up application. Most of these products have a 4 to 6 week waiting interval between application and grass seeding. Check label for the product you use. I mention this as you will probably not want to treat after mid-July for the September planting.

And I will throw out an organic option. Tarp the area with black plastic and let the sun rays kill the plants over the summer. Heat treatments will sterilize the area.

Hope this helps - Dennis - Johnson County Extension

it depends on the things you are wanting to kill