+1 on everything DonMagee said. As for choosing the hops, what aroma are you trying to get? You need to decide that, then look into what hops to choose. For American IPAs, many like Cascade, Centennial, Amarillo, Citra, Simcoe, Chinook, and Columbus, but there are a bunch of others you could use depending on what you're going for. As for alpha acids, many of the great aroma hops tend to have lower levels, but some hops traditionally used for bittering have a high level of aromatics as well and work great for dry hopping. Most of the hops I listed above have alpha acids higher than 6 but are great for dry hopping. Again, choose what you want in the aroma and then research some hop profiles a bit. Hope that helps!