Cassius tells Casca that he already has a lot of supporters for the opposition to Caesar.
Cassius asks Casca to come over so he can talk about the opposition privately. Once alone, he explains to Casca that he has been trying to gather support for opposition to Caesar, and he has already been successful among some very high-ranking Romans.
Now know you, Casca, I have moved already
Some certain of the noblest-minded Romans
To undergo with me an enterprise
Of honourable-dangerous consequence (Act 1, Scene 3)
This is important because it shows that the forces are mobilizing already, and Brutus, Cassius, and Casca have support among the senators. Apparently, Cassius has a lot of support. Caesar is unpopular because he is high and mighty. He has placed himself above everyone else. They are worried that he abuses his power, or that he will abuse his power. Basically, he has not done anything wrong yet, except for being arrogant, but they are afraid that he is too strong and he is going to be too strong to stop before too long. That is why they have decided to act.
The actions happen in this play very quickly. In the first act, Shakespeare is already establishing that Caesar considers himself untouchable, like a god, but that he is actually vulnerable both politically and personally (the mention of “falling sickness,” epilepsy, symbolically and actually makes him vulnerable). He considers himself immortal, like when he doesn’t listen to the soothsayer’s warning. This kind of thing annoys the senators even more.
While some of the senators, like Brutus, believe that Caesar does not deserve to die, they are also concerned about the consequences of allowing him to go unchecked. Brutus decides that Caesar is a serpent, and he needs to “kill him in the shell” before he does more damage.