When building a deck, always nail a thinner member to a thicker member. Hot-dipped zinc-coated nails are a good choice. For more holding power, consider using either ring- or spiral-shanked nails, or go with deck screws. If using screws, it's best to pre-drill pilot holes.
For a high level of security, select padlocks with pin-tumbler locking mechanisms, shackles of high-tech alloy steel to provide pull strength, and special weather-resistant plastic covers to stand up to the elements.