Honestly I think Gaim is the worst possible series to start out with.
Not saying it's bad, far from it. But a lot of what makes Gaim work is that it's a deconstruction of Kamen Rider, and a lot of the great moments come from being familiar with KR in general. I watched Double, OOO, Fourze, and Wizard before getting to Gaim and it all worked in the end. Where those four series had optimistic tone and endings, Gaim was the opposite, going out of its way to tear down the positive. While at the same it tells you to buy the latest Lockseeds. In stores now!
Personally I feel Double or Fourze make better starting points. You get a good sense of what KR normally is and they're just great shows overall.
Also, avoid Wizard. He may look cool, and the fight scene maybe awesome, but it's a slog of a series with boring characters.