You can hire all hiking equipment in Kilimanjaro or Arusha. But if you don’t like to hired stuff you can bring your own from home , and here i will mention all equipment you will need during hiking so you will choose what to bring and what you will to hire from here still:
Day park bag, at least 5pairs of thick socks, 3Tshirt,2 long sleeves, 2winter jackets, 3 stretching trousers,2lt water bottle or camel bag, winter hat, scarf, hiking poles/ sticks, sleeping bag,soft towels,rain coat, water proof hiking boots.