alright after i post about the USB mining many people advice me to buy ASIC
USB miners like Gekkoscience Newpac or Compac F for example, are ASICs, they have a Bitmain chip.
Where are you buying it?
It depends on several things. Where are you located? For example, if you're in the USA, you might want to order from an American reseller, to avoid excessive import taxes and shipping costs.
Generally speaking, ordering directly from the manufacturer is a good idea, but it doesn't guarantee easy delivery. Bitmain, for example, is notorious for sometimes making its customers wait months before shipping their orders.
You can browse the forum, it's full of good addresses and topics on this subject.
You can also order ASICs from reputable forum members, mikeywith has a long-standing excellent reputation, for example.
To cover electricity and see some good earnings how many i need as a start?
It depends on the price of your kw/h. And it also depends on the purchase price of your ASICs. With this data, you can calculate your ROI time and have a more or less accurate forecast (the difficulty changes every two weeks or so, so forecasts are never 100% guaranteed).