Whats best?

here's a good place to start
http://www.allotment-garden.org/articles/Clearing_a_New_Allotment.php
and try the search facility cos this question's been asked lots of times!

You're starting from grass at least rather than a weed-infested old plot - good luck!

Be aware that wireworm is often a problem pest in areas of former grassland.  I can't remember off-hand if there is a chemical way of removing them (suspect not), but what is often recommended is that you grow a sacrifice crop of spuds - they love them and will eat into them.  Don't expect a pristine crop and you won't be disappointed!

If it's just grass, then its probably not necessary to use weedkiller. Mine was all grass when i took it on and just covered it with wet newpaper, wet cardboard and plastic and killed everything underneath that way.
If you're planning raised beds. you could use the no dig method and just bury all that grass under layers. It might not be any harm to turn the soil in the beds over first, even to just break it up a bit and let the birds eat any nasties.

This video is pretty great for no dig potatoes. q1OShZZUt0kHe makes it look SO easy!

Thats a easy way to grow potatoes. I like the hay way.

OK, I've re-read the question now.

For raised beds where the soil is OK you should dig the soil, construct the frame for the beds, and then build the beds up.

If you are on heavy soil you could dig the soil out from the paths between the beds - for 6" or 9" say - and put that on the beds. The lower paths will help drain the beds. I have "lazy raised beds" - I lowered the paths, put the soil on the beds, but I have no rigid frame surrounding the beds.