I will fly from Miami to White Plains, NY, with a commercial flight.
What nearest airport to WP should I choose for my trip?

It depends. As others have noted, White Plains has its own smallish
airport. There are no non-stops from Florida, and fares tend to be
high. You'll pay about $240 from FLL or $300 from Miami and change
planes on the way if you do that. Public transportation there is not
great, so ideally someone would be willing to pick you up.

Jet Blue has very low fares, as low as $160 RT nonstop from FLL to
JFK. American and Delta match some of those fares on nonstops to both
JFK and LGA. You can take a limo (a bus, really) from LGA or JFK to
White Plains for about $40 each way, which runs every two hours and
takes about an hour to LGA or 1:40 to JFK. See http://www.ctlimo.com.
If your friends really like you, you could ask them if they'll pick
you up at LGA.

If you're very price sensitive, you can take a $10 bus from LGA to
Grand Central or the new Airtrain and subway from JFK to Grand Central
for $7, where there is frequent commuter train service to White
Plains.

Don't fly to Newark, which is nowhere near White Plains.

Regards,
John Levine, , Primary Perpetrator of "The Internet for Dummies",
Information Superhighwayman wanna-be, http://iecc.com/johnl, Sewer Commissioner
"More Wiener schnitzel, please", said Tom, revealingly.