Do I necessarily have to buy furniture before I move into my apartment? Can I just buy bit by bit or just buy everything at one time.?

Depends on your budget. If you can't afford everything you want let out the word to family and friends and accept any loans or gifts of used furniture.

After my divorce all I had was dressers, coffee table and end tables so I bought a bed, chair and couch. My parents loaned me a headboard they had and a old table and folding chairs. That worked while I was busy buying dishes and a shower curtain and things to get past the first few weeks. I didn't buy a dining set until I bought a house 18 months later and didn't get a tv until I got a roommate.

No, do what's best for your wallet. If you can afford to spend that much at once, then you should take into account how the furniture will look in the apartment before it's bought. Like, if you have a small living room, a huge couch and two oversized recliners might not be a wise purchase.

You don't need furniture first. In fact, until you know the size and layout, getting furniture may not be a good idea. Then you start with the pieces most important like bed, dresser and chair. Get the rest as you can afford it. Thrift stores, garage sales, and Craigslist are great places for deals.

I once moved into an apartment with an inflatable mattress and a couple of lawn chairs. It took a couple of months to find the furniture I wanted.

It took me two days to find the bed. I mean, (ahem) seriously.

Buy what you can afford. Used furniture can cost less than half price of new and still be nice.

That is 100% up to you. Totally personal preference.