Tilling the garden should be done before you plant your seeds or plants. You need to work up the soil somehow so that it is crumbly and easy to put seeds into. You can plow up the soil, spade up the soil or till up the garden soil to prepare it for planting. Anyway you work up the soil it needs to be done when the soil is ready.
The frost should be out of the ground before you work up the soil. You can check if the frost is gone by taking a shovel or spade and try poking it into the ground. If it goes in all the way then the frost is out and if you can not get it into the ground you need to wait for the frost to leave. Usually a good rain will melt the frost.
The soil should have the proper amount of moisture in it. It should not be too wet or you will just make big clods of dirt that will need to be worked up again later. If the soil is too dry it can be worked up but it is harder to do and it will create a lot of dust. Dry soil can still be tilled and planted but you will need to water the soil well right after you have planted it. You must also keep the soil moist so your seeds can germinate and grow into hardy healthy plants.
To see if the soil has the right amount of moisture in it to work it up you can take a handful and squeeze it into a ball. If the ball holds its shape real well then the soil is too wet. If the soil makes a ball but falls apart then the soil is ready to be worked up. If the soil ball falls apart into dust then the soil is too dry.
Soil that is too wet can dry fast on a windy day. Soil that is too dry can blow away on a windy day and that can be very bad for your top soil. Garden soil that is too dry can blow away on a windy day and take your precious top soil with it. It is important to keep your garden moist since it is soil that has been tilled up and there is no plant matter to help keep the soil in place.