The same rules apply for UK citizens entering the USA from Canada as for entering directly on a flight from the UK
If you've never been arrested, or convicted of a criminal offence, you can travel visa-free, using an ESTA (Electronic System for Travel Authorization) which you can obtain online here:
https://esta.cbp.dhs.gov/esta/
If you don't qualify for visa-free travel you'll have to go through the considerable hassle of applying for a visa (which involves attending an interview at the US embassy):
http://london.usembas...nimmigrant-visas.html
'Routine' applications can take several weeks (because of the need to wait for an interview appointment). However, if you've ever been convicted of an offence of 'moral turpitude' the US Embassy has no alternative other than to refuse your application in the first instance. (The US Immigration and Nationality Act places an automatic life-long bar on anyone with a relevant conviction from entering the USA unless they were only convicted of one offence, while still a minor, and 5 years has passed). The application could then be treated as one for a 'waiver of permanent ineligibility' but you're already too late for travel in July, since you need to allow at least 5 months for such an application. (The longest wait I've seen reported here on AB, was from someone who waited 14 months before being refused a 'waiver of ineligibility' after committing two offences of driving without insurance).
If you qualify for visa-free travel, getting into the USA is relatively easy. If you've been convicted of assaulting someone (which does not normally count as 'moral turpitude') you might still get in after a b