Of course, the best way for you to find out the answer to your question is to read or see the play. There are some very good film editions available as well which can also help. But remember that these are no subtitute to good, rigorous textual analysis. Rather than give you a complete breakdown of the play, I am going to talk you through one of the chief features of the play - the central confusion of love between three characters in the main plot - which you can then go back to and "flesh out" with your own reading.
Typically as one of Shakespeare's comedies, this play involves mistaken identities, cross-dressing, disguises and people falling in love with people they shouldn't fall in love with. Thus it is that we are introduced in Act I scene i to the Duke Orsino, who is swift to declare his undying love for Lady Olivia:
O, when mine eyes did see Olivia first,
Methought she purged the air of pestilence.
That instant was I turned into a hart,
And my desires, like fell and cruel hounds,
E'er since pursued me.
This situation is immediately complicated by news of Olivia's unremitting resolution to not marry for love of her dead brother:
The element itself, till seven years' heat,
Shall not behold her face at ample view,
But like a cloistress she will veiled walk...
So, it is clear that Olivia does not return these feelings, as we see from her distaste of being forced to listen to them once more from Cesario. However, this situation is further complicated in Act I scene iv by the irony of Viola dressed as Cesario being forced to take messages of love from her master to Olivia when she is in love with Orsino herself!
Yet a barful strife!
Whoe'er I woo, myself would be his wife.
Then of course note Olivia's response to Cesario in Act I scene v:
Even so quickly may one catch the plague?
Methinks I feel this youth's perfections
With an invisible and subtle stealth
To creep in at mine eyes.
So, three characters, each involved in a crazy tangled up relationship involving disguises, secret loves and unrequited love. The real question of course is how on earth is Shakespeare going to resolve this situation in this "comedy". Now, as you read through the play, make sure you pay attention to how the subplot relates to the overall plot - how do characters such as Sir Toby relate to the confusion in the main plot? What about the relationship between Sir Toby and Maria? This will help you think through how the subplot and plot relate in this great play.