Understand the scale before you plan anything
The single most common mistake here is treating Phuket like a resort rather than an island. It is large, hilly, and spread out, and a plan that involves breakfast at one beach, an afternoon at another and dinner in the Old Town will consume most of the day in vehicles.
Tourism Authority of Thailand's own characterisation of the island is a useful shortcut: the south coast beaches are typically crowded, the north far more tranquil. Where you have booked determines what your default day looks like more than any itinerary you write.
Sort out transport on day one
Work out which options actually serve your area. The Phuket Smart Bus runs the airport to Rawai corridor via Patong, Karon and Kata with a flat 100 baht fare, hourly, on published daytime hours, plus a separate flat-fare route between Bus Terminal 2, Phuket Town and Patong.
Then work out your evening plan, because the bus finishes in the late afternoon. Ask your accommodation how guests usually get back at night and whether they run a shuttle. Doing this on the first afternoon, sober and unhurried, is worth a great deal at eleven that night.
Before your first swim
Learn the flags. Thai disaster prevention guidance sets out two red flags as a complete closure with no water activities, one red flag as dangerous with swimming prohibited, yellow as caution, and red-and-yellow as a lifeguarded area where swimming is permitted. It also advises staying out of the water during and after heavy rain because of rip currents and shore break.
This is not boilerplate. The Andaman coast produces strong rip currents, and the water that looks calmest between the breakers is often the current itself. Swim in patrolled areas, and if a red flag is up, treat the pool as the plan for that afternoon.
The practical checklist for day one
Two rules worth knowing immediately
E-cigarettes and vaping devices are illegal to import into or possess in Thailand, with published penalties including imprisonment and fines based on the value of the goods, plus a fine for using one in public. Phuket's tourist areas are not an exception to this.
The other is about water sports. Rental disputes over alleged damage to jet skis and other equipment have a long history on this island. Photograph the equipment before you take it out, film a slow walk around it, and be clear about what you are agreeing to pay for. Most operators are fine; the ones that are not rely on you having no evidence.
Settling in
Once the first day is done, Phuket becomes an easy place to be. The infrastructure is good, English is widely spoken, and the food ranges from beach shacks to some of Thailand's better restaurants. What it asks of you is the same thing every large island asks: plan around the distances rather than against them.