No style is any more or any less effective than any other style. What matters in a fight is how you react and how you control the situation. You develop those skills by training with good guidance. If you can find an instructor who knows what he or she is doing and you put a lot of time and...
Every modern style of Karate has been developed within the past 110 years. Many Shodai Soke even had the same instructors. The many styles of Karate that we see are the result of different philosophies and personal tweaks on the same material. In the end, no Karate style is better than any...
Which military? The Israeli military uses Krav Maga, the Chinese military uses Sanda, the Thai military uses Muay Thai, the USMC has its own system, the JGSDF uses Karate. You've really described most martial arts.
You get your white belt on your first day, but I suppose you mean your first rank up. You earn a new rank when your instructor thinks you're ready (usually following a test). Belt systems vary from style to style and from dojo to dojo.