Most students will be applying at the beginning of the second year of sixth form for a place the following autumn term. Because of this, universities will scrutinize GCSE and AS/Highers results together with predicted A2/IB/Highers marks.
Unlike British universities, acceptance will not be dependent on your A2 etc. results - these are reported too late to impact acceptance in the US.
Don’t think this allows you to do nothing for your final year. Your final grades will be reported eventually and you don't want to get off on the wrong foot. Or have your offer withdrawn at the last minute (it's happened)!
The least you need to apply to a US university is five GCSEs, or Scottish Standard Grades, at C or above in core subject areas, including English and maths – a hurdle you will leap lightly over.
This is not to say that at the age of sixteen, laurelled with five Cs, you will be welcomed with open arms by Harvard. Naturally, the better the uni, the better your grades need to be.
All universities normally want a student to be eighteen and above, unless there are very unusual circumstances, and most universities will want to see post-sixteen attainment too: generally two to three A levels or equivalent (International Baccalaureate (IB), Scottish Highers, BTEC, GNVQs, AVCEs). The best ones won't open your application if you haven't gone further than GCSEs.
All of these will be considered, but it is important to check with individual universities to see what their minimum standards for admission are. IB in particular is well understood. In rare cases you may need to pay to have your qualifications evaluated – see the References and Links section for companies that will do this.