how to prepare a raised bed for vegetables?

Here you go.

http://organicgardening.about.com/od/startinganorg...

Unless you have super compacted soil, you should be able to build it directly on top of the grass without a problem. Most grasses can't penetrate 2" of soil, let alone the 6-12" used in most raised beds. Roots from your garden plants will still be able to penetrate the soil below because the soil on top will keep everything underneath it moist, and the decomposing grass will attract earthworms and insects by the thousands, which will aerate and loosen the soil, as well as fertilizing it.

When I build an 8" raised bed in the fall (I'm assuming you're in the northern hemisphere and plan on building it soon), I fill the lower 4" with grass clippings and dead leaves, then add maybe 1" of either sand or wood ash mixed in because it changes the soil texture and breaks up the small clumps in the soil that often form after all that organic matter decomposes. From there I top it off with compost or topsoil and let it sit for a week, then cover the whole thing with a mulch like straw, sawdust, etc. after everything starts to sink from rain and the decomposition of the organic material.

By the time spring rolls around and you're ready to start planting, the soil will have sunk a few inches and earthworms as well as a slew of other invertebrates will have aerated and mixed the bottom 4-6" or so of soil fairly well.

Take out the Grass in a area beyond the raised bed, Tack on some Chicken Wire to the bottom of the wooden bed so Ground hogs, Moles and or Vols do not come up to steal your bounty. in the area around the bed which you have taken grass from lay day Straw or other material to form a path around the bed, this will hold back weeds and make it easier to access the bed in general. If your not going to make a wood frame, then it's still a good idea to remove the sod as mentioned above.

You can spray the grass with roundup and after a couple days cut it really short and add the soil. You'll want to add at least 6 inches if not more and I would use mostly compost. I have done this before and it worked great! You may need to add a little lime in the spring because even though the compost provides lots of nutrients it can be a bit too acidic after a little time. It is much easier to work with less topsoil in it though.

Put a layer of cardboard, or about eight sheets thickness of newspaper, on the grass and put the soil mixture of soil, compost and vermiculite on top of that. The cardboard and soil on top of the grass will kill the sod. Then the cardboard will rot down to permit the roots of your vegetables to pass through.

Depending on the type of vegetables you are using, then I would remove the grass and slightly bust up the dirt before you put the box and new dirt on top. Some plants roots like to go a lot deeper than what you will be making the raised bed, so you will need to remove the grass and bust up the dirt, so that the roots can go as deep as they need to go.

I'd build it directly over the grass...if you're making your bed 8-10" deep, that grass will never make it outta there alive.....and the rotting green stuff will add nitrogen to the dirt...Test ur dirt's Ph when finished filling the garden in and adjust by adding the appropriate admendments.

Your best bet is to buy some good quality sterilised top soil at least 300mm deep onto of your already turned soil. I know its not always cheap but its so much better. Then you only need to try and keep out weeds that blow in on the wind.