I'm assuming you're referring to the confinement week of NS.
You should definitely consider getting some comfort items. This can be the pillow that you've used for the last decade to fall asleep on, milo to drink before you sleep, power banks (no charger for BMT) for talking to your loved ones more or intensive gaming, etc. Refrain from bringing in big items like kettle or laptop (depends), and potentially messy food like instant noodles and sugared/chocolate coated-food. There's no need to bring in vitamin c tablets since there's fruit provided every meal.
For daily use items, you can bring more clothes hanger (maybe 7 or more) to hang your clothes. Bring in a used cloth/rag for simple cleaning around your area (environmentally-friendly!). You can get a pair of slippers/sandals to move around your bunk and for showering. If you are prone to muscle soreness, you can get some muscle pain relief patches/spray. Deodorant/de-odouriser even if you don't smell bad in case your buddy does.
The first two weeks are packed with PT and administrative activities especially if you are entering as combat fit (1yr10mths). During this period, your unit will also clarify what can and cannot be brought in for future book-ins.