Thank you everyone for answering! i followed the advice of skielbasa and TRac..i've just bought the front and rear whole wheels from an ebayer that sold them to me for 65€ and he also included the cog
http://www.ebay.it/itm/251299673783?...#ht_914wt_1190
certainly they are not a good quality wheels, but I think they are good for starting then when i'll get more money i will make better quality wheels.
Now i have to buy transmission, pedals and bottom bracket 'cause in the frame i'm using there was this transmission that i don't know the name in english


and i thing the bottom bracket it's wrong..isn't it?