Welcome to the NerdWallet community, love_nicola! And congratulations on the work you’ve done to build your credit thus far.
First, whether something helps your credit will depend on whether it’s reported to the credit bureaus. If you choose to finance through a furniture company, it would be smart to be sure the loan is reported AND to check the interest rate vs. what you might pay elsewhere (some have pre-qualification that lets you know your chances of approval without your applying — because actual credit applications can cause a small, temporary dip in your score).
You might get more benefit from applying for a “credit-builder loan”. That would give you an installment loan (one with level payments for a set term) as well as a credit card, and different types of credit can help your score. But the two things that help most are paying on time and using just a small part of your credit limit. (Using less than 10% is even better than staying under 30%.) In general, the lower the better, as long as there is at least some activity on the card.