1st off, figure out how deep you want your beds - you'll need at least 6" if oy plan to do carrots/parsnips and more for tatties, but most other things should get by with 6"
mark out your beds with some string and i would recommend you dig over the ground yo uplan to put them on, add plenty organic matter too - this facilitates drainage, attracts worms a plenty too.
Get a fence post - anything over 2" by 2" and cut it to the depth you want your bed - plus about 4-6"
i actually bought some fence posts that were pointed at the end - like stakes for slaying vampires!!! that were about 18" long, these were great
knock the fence posts/stakes into the ground at the corners of your raised beds
screw the planks that will make up the walls of your raised beds to the fence posts.
get some wood treatment and treat the wood for your beds and let it dry. it's probably also worth lining the inside of the wood with polythene (to stop the damp soil rotting the wood)
shred some paper/card line the beds with that and then fill your beds up with compost/soil and you are done
hope that helps
of course, some folk just pile earth up, some use tyres, you can use sleepers (but don't use ones that have been treated with creosote)
you can even build brick ones which are pretty - but that would take a bit of skill/time/money!!!