The first thing to do is get yourself on a 'Competent Crew' course - this is what the RYA (Royal Yachting Association) in the UK calls it. Other countries sometimes give it a slightly different name but generally it'll be an entry level course from the main yachting association designed to give you your first grounding in the general principles of being a solid crewman.
Once you have that nailed you'll need to get yourselves on the water as much as possible. The cheapest way of doing that is usually joining a sailing club if there's one near enough for you, or offering your newly minted services as an able crewman and trying to get on board by working as crew rather than paying for the honour.
We also strongly suggest you get a lot of dinghy sailing experience in. This will get you comfortable with being constantly covered in spray, sitting next to the waterline and the more responsive nature of tiller sailing as opposed to a big steering wheel.
Once you've had a bit of time on the water the minimum qualification we need (along with experience and time on the water) is Day Skipper - another RYA course as it's named in the UK.
This course is designed to make sure you're able to sail safely and proficiently for a full day and crucially includes the ability to navigate so you don't end up drifting off into the Indian ocean when you take on the race.
RYA Competent Crew Course info
RYA Day Skipper info