You may purchase your armor, weapons, and shields from the relevant tables, but you carry any leftover gp into the game. (Weapons, armor, and magic items are considered declared items, not supplies.)
At the GM's discretion, you may start with a single magic item, rolled randomly from the treasure table.
The number of slots for declared items in your inventory is equal to your Mighty +10. Supplies do not take up slots in your inventory, nor do items that are equipped to you. For example, if you're wearing armor or wielding weapons, these items do not occupy slots in your inventory.
If you're playing on pen and paper, each item occupies 1 inventory slot. The GM, of course, has final say as to whether what you're carrying is reasonable. Sixteen broadswords is not reasonable to have in your inventory.
Slots in the Character Creator
If you're using the Character Creator to manage your character sheet digitally, then managing your inventory is easier, but more granular: any items you declare will automatically occupy an appropriate number of slots for their size, per the table below.
Item Size in character Creator
|Tiny||0||Feathers, coins, rings, gems, candle, soap|
|Small||1||Handheld mirror, book, rope, potion, lantern or torch, a day's rations, scroll|
|Medium||3||Sword, shield, backpack, quiver, helmet, shovel, bedroll|
|Large||5||Suit of armor, rolled up tent, chair, bear trap, 1000 gold|
|Bulky||7||Wagon wheel, cauldron, barrel, bathtub|
When you carry more items than you have slots in your inventory, you are encumbered. You receive disadvantage on all Mighty, Deft, initiative, and defense checks, and move at half your movement rate (a single personal space per round rather than a melee space).