The main question is what sort of visa are you looking to get and what is your nationality?
I'm working here on the assumption that you're a British citizen.
Brits can visit the USA for up to 90 days at a time without a visa.
Are you looking to get some sort of visa that will allow you to work in the USA or are you perhaps the spouse or fiancee of a US citizen?
There is an exception for people who have only committed one minor offence.
"...for a summary offence" I believe that an individual summary offence in the magistrates court has a maximum sentence of six months.
The US law says that if a person applying for a visa has been convicted of a single offence for which the maximum penalty is no more than 12 months in prison and, if you did go to prison, then you weren't sentenced to more than six months that this person is still admissible to the USA with a visa.
However, this doesn't apply to drugs convictions - a single conviction related to drugs and they don't want to know at all.
The relevant law is 8 USC §1182 (a)(2)(A)(ii)(II)
"(II) the maximum penalty possible for the crime of which the alien was convicted (or which the alien admits having committed or of which the acts that the alien admits having committed constituted the essential elements) did not exceed imprisonment for one year and, if the alien was convicted of such crime, the alien was not sentenced to a term of imprisonment in excess of 6 months (regardless of the extent to which the sentence was ultimately executed)."
8 USC 1182: Inadmissible aliens