The closest airport to Tumon Beach is Tamuning Airport, which is located in Tamuning. If you plan on visiting or staying in this area, landing at Tamuning Airport will be your best bet.
The best prices found on momondo for flights to Guam are:
$104 per person with T'way Air
$119 per person with Air Seoul
$135 per person with Jin Air
Guam has restricted the entry of all non nationals/residents who have been in a country with confirmed COVID-19 cases for more than a week, and do not hold a document certified by DPHSS (Guam Department of Social Health and Services) that confirms they are not infected with COVID-19. The date of the test must not be more than 72 hours before the date of arrival. Travelers who enter Guam without proper documentation will be quarantined. If you are planning to travel to Guam at this time, it is recommended that you stay up to date on current restrictions and follow proper safety measures while in public.