It depends on how you want the grapevine to look when it's fully grown. If you want it as an ornamental type of a vine, say, to look nice in the back garden and provide a bit of shade in the summer and are not too worried about quantity/size of grapes, then let it do it's own thing and keep growing. I would say that whatever you do want it to be, it definitely needs a larger pot or putting direct into the ground in late autumn when the leaves have dropped.
One way of starting to train it would be to examine it, and choose the thickest and most vigorous stem emerging from the soil, and cut out all the rest. (assuming yours is multi stemmed at the moment, I can't really see). There is a lot of growing still to do this season, so that stem will get longer and thicken up. Probably you'd want it to continue getting longer during next year too. Once it gets to the height you want (and the height you choose will be influenced by where it's going to be supported) then cut it just above a leaf node. This should stimulate that stem to fattening up a bit more, and also you will find that sideshoots start emerging from it, and also it will throw out new growth just around where you cut it. At this point you can start to manipulate the shape of the vine.
Have a look at the pictures on this link, which show a youngish vine which has been trained fairly clearly:
http://www.honeyflowfarm.com/grapeproject/grapeproject.php
