Hrm. Maybe start off with the Marvel Ultimate books? While they're up into the 100s on some of them (Spidey, X-Men), they don't have nearly the continuity pressure as the regular universe does. Also, they are paced for the TPB market, and you can easily stop off at a local bookstore and read one or two at a time to get a feel for it.
Beyond that, pick something up at a # 1. Even if it has continuity behind it, it's still a relative starting point and they do try and have new readers come on. Also, there will be explanations along the way in many books, as they do assume new readers come on. That's why (for example) Psylocke's mind-blade was described every other issue as the sum totality of her psionic might, etc.
Brad