Okaaay...
1) On the current goal of the Heroes...
There is never just one collective goal; there are always many, individual goals. I'm not going to go into all of them, because it's not that hard to follow. A few include: Stopping The Bomb from destroying New York (everyone), stopping Sylar from killing more people (Mohinder, Hiro), disabling the Company's ability to track evolved humans (Bennet, Parkman, Ted), and ending connections with Mr. Linderman (Niki, DL). There are lots of other goals that happened in the first season. Just pay attention.
In the second season, we're currently dealing with the issue of whomever is killing the older generation of Heroes and the Company's messing around with the Shanti Virus.
2) On the issue of Mr. Bennet...
We know exactly what he wants. He wants to bring down the Company and to keep his family safe. He is doing it in whatever way he can.
3) On the issue of the Haitian...
...well, what about him? We don't know his connection to Angela Petrelli, though I'm assuming it has to do with the Company, but other than that...I don't see what there is to not understand about him.
4) With Hiro being in the past...
Hiro doesn't belong in the past. He is from 2006, not 1671. His presence had screwed up the time/space continuum; his hero, Takezo Kensei, turned out to be an alcoholic bum rather than a mighty warrior. A town that Kensei reputedly saved was burned to the ground. Whitebeard has ended up having firearms. Hiro wants to set everything right, though he can't help his emotions and ends up screwing things over more by falling in love with Kensei's 'princess.'
5) How did they get their powers? Evolution. Biology. Tada! It has something to do with the solar eclipse. Other than that, there's no real answer and one isn't needed.
6) Why do they keep adding 'stupid' characters? Because it's a television show. Originally, season two was going to have all new characters. No Hiro, no Peter, etc.