5) The Sith are force users who use the dark side of the force and give into it's power. There's a rule called the rule of two that states there can only be a Sith master and a Sith apprentice. Their ultimate goal is to rule the galaxy (which Sidious accomplished).
2) The Senate is the representation of planetary systems in the Republic. If you'll notice, each of the little pod things is for a system and the alien race represented in it. It makes all the laws for the Republic.
1) I assume you're referring to the Trade Federation. I'm not sure exactly what they did before, but they had a substantial army of droids, and Palpatine used them to invade Naboo make the current Chancellor look weak when he didn't do anything, that way, the Senate would vote Palpatine into power (he was from Naboo, which is why Queen Amidala wanted him as Chancellor in the first place).
4) Count Dooku was really in charge of that (but ultimately his master Sidious ran the show). He organized different systems from the outer rim who for some reason or another wanted to be free from the Republic. Dooku got Grievous to be the general for the droid army.
3) As explained in 1, he became Chancellor as a result of the events on Naboo. He used the start of the clone wars to use his emergency powers (that Jar Jar voted to give him while Padme was away) and stay chancellor longer than he would normally be allowed. Because of the clone wars, he had an excuse to use the clone army which was genetically programmed with certain orders (notably order 66) that only he could initiate. From what I understand, his plan with the clone wars was to make the people in the senate want to give him more power so he could establish security throughout the galaxy. After defeating Windu and the other jedi who tried to arrest him and making Anakin his apprentice Darth Vader, Palpatine ordered that Order 66 be carried out, which was that the clones should exterminate every jedi. After he ordered that, he went to the Senate and officially declared that he would be reorganizing the Republic into the "first galactic empire", thus accomplishing his goal. In episode IV, we find out that he eventually dissolved the Senate altogether.
I hope that explains it, I love star wars so it's always fun for me to answer questions :)
