Entry to Langkawi is just entry to Malaysia — there's nothing island-specific about the visa. Most nationalities get in visa-free, and nearly everyone must file the free Malaysia Digital Arrival Card (MDAC) before arriving. The full rules live on the national guide; the one thing worth flagging here is that the arrival card applies however you reach the island — including the ferry.
The short version
Most Western passports get a visa-free stay of up to 90 days, your passport needs six months' validity, and the MDAC must be submitted (free, on the official immigration portal) within three days before you arrive. None of it is different for Langkawi than for Kuala Lumpur.
Note
For the visa-free durations by nationality, the MDAC step-by-step, passport rules and the Sabah/Sarawak border quirk, see the national guide — it's the source of truth and kept current: Malaysia visa & arrival.
The one Langkawi wrinkle: arriving by sea
If you're reaching Langkawi by ferry from Kuala Kedah, Kuala Perlis or Penang rather than flying into the airport (LGK), the MDAC still applies — it's tied to your entry to Malaysia, not to the airport. If Langkawi is your first entry into the country by sea, have the confirmation ready before you board. If you've already cleared immigration on the mainland, the domestic hop to the island needs nothing extra.