Various species of small aquatic worms make up the core diet of betta fish in the wild. The most common worm for betta fish is the bloodworm, which comes live, freeze-dried, frozen, or in a gel, but these aren't very nutritious and are better used as treats. Brine shrimp is a good option or glassworms (tubifex worms), but specially made betta pellets and gels are best.  Live tubifex often carry parasites or bacteria, so they should be avoided. The best live worms to use are white worms, grindal worms, and black worms. These worms are available at most major pet stores. You can use either live or frozen insects. The best options are Daphnia, also known as a water flea, and fruit flies. These insects will be available at most pet stores. Flightless fruit flies are often sold live in jars for reptile use but are also useful for feeding fish. To serve the flies, shake some into a plastic bag and place in the freezer for a few minutes. This will slow down the insects. Then, quickly pour the flies into the tank. Scoop out any that are not eaten. There are various frozen meats that betta fish can also eat. You can use brine shrimp, mysis shrimp, or frozen beef heart. These options will be available at most major pet stores. Beef heart and rich meats can dirty the tank with oils and proteins, so this should be a rarer treat.

TL;DR in Vietnamese:
Cho cá ăn giun. Cho cá ăn côn trùng. Cho cá ăn những thức ăn khác.