To buy household items etc. beyond what you can get in Porter Square, you may need to pay a visit to Watertown Mall, with a large Target store, and many others. You can take a bus from Harvard Square there (line 70 and 70A). A closer Target store is in Somerville, although it is smaller and less accessible. It’s also worth visiting the Family Dollar store next to the Target in Somerville. It’s the ultimate discount store.
In any case, most stuff can be bought on Amazon (remember to sign up for Amazon Prime, free for students for 6 months – but turn automatic renewal off, or they’ll charge you once the period is over).