So what do you actually need? Well a baby needs to be warm, fed, changed, loved and to sleep. Those are the things it actually needs. So with that in mind:
Clothes - well, yes. You'll need lots of vests, babygrows and hats. Depending on the time of year, they might also need a snowsuit to snuggle in for being outside (but this should be removed when they're in their car seat - see some guidelines on that here). It's always difficult to know if baby is warm enough, the best way to tell is to feel their chest or their back, it should just feel the same warmth as you.
Food - if you're planning to breastfeed you've already got everything you need. If you're going to formula feed then you'll need formula, bottles, teats and a way to sterilise. You can get sucked into gadgetry here but you really don't need to. The easiest way to keep bottles sterile is to buy the traditional sterilising tablets and have a bowl in the kitchen with it in (replaced every 24 hours), and put bottles into it after you've washed them after each use. Then you've always got a sterile bottle ready to go, at the right temperature. If you buy a microwave steriliser, they are great but if you need a bottle NOW (which can happen!!) then it will be too hot straight after use and you'll have to wait for it to cool down. Ever tried explaining to a hungry baby that it needs to wait patiently???!!!! And I will sound like my Mother now, but room temperature is right for baby as that is how your milk comes if you were breastfeeding. If you ever offer baby warmed milk you're really setting yourself up for trouble - be warned!!!!
Also in the food category I would say Muslins - you cannot have enough muslins, you'll need one in every room and one in your changing bag. Baby's throw up, often on you when you're out! They also dribble whilst feeding or just after, having a mussy tucked under their chin or to hand to mop up is great. Buy lots of these!!!
Changing - The bag. God people spend £££££'s on changing bags! I've always found that a nice big handbag with lots of pockets is fine, it doesn't have to be an official 'changing bag' to serve it's purpose!
You'll obviously need nappies (buy shares in these!), some people prefer cloth and some prefer disposable - it's your choice. Warm water and cotton wool is the gentlest way to clean baby's bottom, but this isn't practical when you're out so you will need wipes if you ever want to leave the house. I do strongly suggest buying a tub of vaseline, if you apply this generously to baby's bottom and bits after each nappy change, you will avoid nappy rash - it also means that poo doesn't stick to their skin and comes off easily. You're welcome.
Sleeping - Again this comes down to choice, but baby is safe to co-sleep with you under usual circumstances, and you cannot spoil a newborn baby. Just because they co-sleep with you for the first couple of months of their lives (or longer), doesn't mean that they'll be 12 and sleeping in your bed. However it is important to be armed with the facts on co-sleeping, and you can find those with the Lullaby Trust. There are lots of options to have baby in your room and very close to you if you prefer not to co-sleep, such as a Moses basket next to your bed.