new gardener

Hi all as you can probably gather from the title of this new thread I'm quite new at this, we have a extensive garden but most of it has been already been shaped to what we like so far ie large rockery rebuilt another few trees planted and borders sprused up . What we are looking to do now is use a little piece of land roughly 4x4 behind the shed that receives a good amount of sun from 1pm onwards as its on the north side of the house, don't get me wrong the whole of the garden gets more than it's fair share of sun starting in the south of th house and working it's way north it seems. We have just planted three lovely trees one of which is a fruit tree a damson which is doing rather well that is more northerly than the other two one is more easterly and the other in between. We would like to create a little plot of land for growing fruit and veg the soil is reasonable a bit of clay here and there and a very large oak tree at the back of the garden by the back door but most things grow really well the latest being lovely sunflowers from seed it seems they've just shot up from nowhere and all the same height so I would like some suggestions about what to try first are potatoes very difficult to look after many thanx for any imput. Jimmy and lisa