Mantis will mostly feed on anything! They can eat crickets, locusts, blue bottle flies, green bottle flies, fruit flies, curly wings flies, butterflies, moths including wax moths pupated from wax worms, firebrats, silkworms, phoenix otherwise known as calci-worms, soldier flies pupated from phoenix worms, cockroaches, mealworms, morio or super worms, maggots and as a treat a little blob of honey.
Note about crickets: be aware they can bite and carry parasites. They shouldn’t be fed to floral species like orchids, thistles, idolomantis, cryptic, violin, spiny flowers or anything with flower in its common name. There is something in the gut bacteria of the cricket that doesn’t mix well with these species and should be avoided. Locusts are a much better alternative to crickets. All of my mantis are raised on fruitflies, bluebottles, wax-moths and locusts.
In general you want to give your mantis prey items that are no bigger than 2/3rds of the size of your mantis. Some mantis like my L4 orchids will take blue bottles the size of them. Species like Rhombodera and Pnigomantis are super aggressive feeders and will easily take on prey their size which is quite something to watch.
To avoid the risk of injury however, opt for prey items about half or a 3rd the size of your mantis.
A well fed mantis will have a plump or bulbous abdomen. A mantis in need of a feed will have a flat/skinny abdomen. You should feed at least twice a week or every other day for very young nymphs. You could feed multiple small prey items like fruitflies all in one go, quantity will depend on the size of the mantis. Aim to have a nice plump abdomen but try to no over feed all at once. This sounds like a bit of a minefield to a newbie, I know. Your'll get a feel of what prey items fill your mantis up and stagger the feeds. Sometimes overfeeding can lead to a split abdomen (which mainly happens with to female big african/asian types that over engorge) or vomiting.
Example - Adult Giant Asian female will take a subadult locust 3 or 4 times a week.
L3 Ghost nymph will take 6 fruitflies in one sitting 3 times a week. A hotter kept nymphs' metabolism will burn off that food reserve quicker than one kept at a lower temperature. Feeding is can be very subjective. Read your mantis' abdomen. If its thin...give a decent feed. If its plump, offer small scattered feeds in a couple of days. If its mid-full then give a decent feed and not feed again for a few days ;)
Note: If your mantis goes off its food then its possible its either too full or going into moult soon. You should remove all food from the enclosure as food items can disturb or even bite your mantis whilst moulting... leading to fatal consequences.