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The very best time is in the spring, when temps are cool and rain is at its most plentiful. Fall is the second best time, for nearly the same reason.

This article will help you with amending the soil: https://www.gardeningknowhow.com/edible/vegetables/vgen/vegetable-garden-soil.htm We have put together a guide for new gardeners too, if you are interested in that: https://www.gardeningknowhow.com/edible/vegetables/vgen/starting-vegetable-gardens-guide.htm Here is an article that you may find helpful as well: https://www.gardeningknowhow.com/special/starting/how-to-build-a-flower-bed-starting-a-flower-bed-from-scratch.htm

If you have it, composted manure can get the nitrogen in the soil very quickly and will also help plants with taking up phosphorus already in the soil. Simply add it over garden area and then this spring till it (along with cover crops) into the soil. Likewise, you could just leave as is for now, till it in spring and then add additional soil additives (like the composted manure) during planting.

There would be far too many to name, but most plants are ready for planting once the threat of freezing temps is passed and it stays at least 50 degrees F. day/night. Generally, this is dependent on your location and the type of plant you are growing. These articles may offer some additional information that will help: https://www.gardeningknowhow.com/edible/vegetables/vgen/when-to-plant-your-vegetable-garden.htm, https://www.gardeningknowhow.com/ornamental/flowers/fgen/early-spring-flowers.htm

This article should help: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/how-long-does-weed-killer-last-in-the-soil.htm Almost all weed killers and pesticides have a period of time after which it is safe eat food grown in them. The time varies as to how long the soil may still have remnants of the chemicals used. After about a year without the use of any chemical control, however, it should be ok. As a precaution though, I would suggest washing any fruit or veggies thoroughly prior to eating. If this garden is simply a flower garden, then the period of time would be less since the plants would not be ingested. Of course, most people don't want to wait that long, so you could instead implement raised beds in that area. This would allow you to begin growing your garden this season. For more tips on gardening in rasied beds, this article will help: https://www.gardeningknowhow.com/edible/vegetables/vgen/raised-vegetable-gardens.htm

You do not need to remove the soil. This article has direction on how to do this: https://www.gardeningknowhow.com/plant-problems/weeds/how-to-kill-grass-naturally-kill-unwanted-grass-in-your-yard.htm You can also choose to go ahead and till it into your soil. Any large pieces of unearthed sod can then be easily removed and tossed.