There are Storage Manager NPCs in most Villages who can help you manage your Inventory by providing extra storage space. 

There are Normal Storage and Account Storage. 

All items can be stored in the Normal Storage with 50 slots and a maximum of up to 250 slots can be extended.

Only 10 slots are provided to the Account Storage and there is no option to extend extra.

The items stored in the Account Storage can be shared with characters on the same account, but you can't store items bound or certain items.