This is a great question because it identifies the different plot strands that make up this amazing play. The first of the two central plots are Bassanio's quest to gain Portia in Belmont and his success in picking the correct casket and thus gaining her hand. The second central plot, obviously linked to the first, is the loan that Antonio is forced to take out against the ships he eagerly awaits to furnish Bassanio, his former lover (though this is debateable), with the money that he needs to enter Belmont and win Portia's hand. This of course puts him in conflict with Shylock and ends him up in front of a court with a bloodthirsty Shylock ready to shed his blood.
The two sub-plots concern Jessica's elopement with Lorenzo and their removal to Belmont and also Launcelot's decision to leave the employment of Shylock and join his new master, Bassanio. Of course, it is important to identify how the sub-plots link in with the main plots - for example, how Jessica's desertion and robbery of her father's money is used to goad Shylock on to gain his revenge.