Congratulations on the lead!
– you don’t charge VAT on exports of goods (generally)
– you need to decide (agree with client) whether to require payment in GBP or USD. USD will be more attractive/easier for them but GBP will be less hassle for you
– you need to decide (agree with client) who’s going to arrange and pay for shipping, customs fees, US duty/taxes (if any) etc
– you need to think about possible after-sales costs e.g. warranty returns (depends on product) – go through whatever normal T&Cs you have and think “what happens if they’re in the US?”.
In addition to what TooTall says, http://www.businesslink.gov.uk/ may also be useful (if you’re in England) and even your bank might run small business exporter workshops etc.