Add set ic (set ignorecase) to your vimrc to have search ignore case by default.

Use case sensitive search only if a capital letter is used in the search set smartcase

Set them in the editor temporarily in command mode: :set ic or :set smartcase

To turn them off use :set noic or :set nosmartcase

Toggling the settings could be useful as well with :set ic! and query its value with set ic?