A card formatted to FAT32 (preferably 2GB or smaller for older firmwares, though up to 32GB works for 4.0+).
If you are looking to mod your Wii, the community currently recommends these official sources: Wilbrand (Web Version): Available at the Wii Hacks Guide
Launch an instance directly from the terminal:
A: Yes, Wilbrand is widely considered safe and is the recommended method for installing the Homebrew Channel on modern Wiis.