Reasonably easy, if you really cant do it i'm sure someone will assist you or a shop will charge, not sure how much though, £30 maybe ??
mine....a little more xc than DJ compared to Matts above ^^ but still more than capable....
Its really hard to choose, the best advice is to just sit on as many bikes as you possibly can and try to work out which size of frame feels most comfortable to you and make a note of its size and use that as your start point as it will eliminate loads of 'maybes' with the size sorted you can start looking more at geometry a jump bike will have a lower top tube and different head tube angle compared to a XC bike for example but that brings with it different characteristics, like how it behaves riding up hills for example how long the seat post will need to be to make it comfortable for you etc
then you can start looking at brands/models and reading reviews looking at what hardware they come with, no good getting a great frame thats your size but has cheap nasty forks you'll break ! you'll have to compromise somewhere, thats why i built my own bike so i could compromise where i was happy too rather than where a manufacturer thought i should !