You have two separate, rather large questions here, so I'll do my best to get to both of them in limited space.
Money is important in elections because it buys you access to the public. Ads on television are very expensive and you have to advertise nationwide, so this expense runs into the tens of millions of dollars. Mainly for this reason, the candidate with the most money tend to win. This has been true in the last three election.
To campaign nationwide requires a very large staff covering 50 states, all of their offices, copy machines, cell phones and hotel rooms, and usually a 747 jet to carry that staff around from campaign stop to campaign stop. To fire the best staff workers and campaign managers can also run into the millions of dollars.
As far as how candidates reach potential voters, there are several methods of varying effectiveness. Television is the most convenient, as almost every American has one and watches one. Radio and newspapers can also work, but once you get past the basic media, "All politics is retail" as was once famously said. This means door to door campaigning, meeting and shaking hands with thousands of people in key states before the primary and general election, and participation in town hall meetings and debates.
To see how much money we're talking about and where it comes from, visit the link below to The Center for Responsive Politics (non-partisan).