Work in some good rotted horse/cow manure, some compost of organic matter. You'll be good to go.

If you buy topsoil be careful where you get it - some builders will sell you rubbish. You haven't put your location on your profile so perhaps you can do your own search - just google Topsoil and your county and you'll see a map or what I hope are reputable suppliers. Then as Bathgate suggests you can improve it with a bag or two of well rotted manure and some compost if you have any (not potting compost)
Then it may be worth your while to buy a soil testing kit to find out if any extra nutrients are needed. Or you can just add some Growmore to be on the safe side. It would be useful to know the pH of the soil (ie how acid or alkaline it is) as different veg have different preferences.

And if you want to grow root vegetables, some sand will be useful in a section or two.