Де́вочка is still a child, де́вушка is not. So, a 10-year girl is де́вочка, a 20-year girl is де́вушка. The distinction is pretty blurry, but I hope you get the general idea.
(However, sometimes the word де́вочка is used metaphorically when speaking about adults. For example, older women might call each other де́вочки.)
Unfortunately, this option is only available for some courses, and Russian is not one of them.
You can see the conjugation of verbs in the Wiktionary: https://en.wiktionary.org/wiki/есть#Conjugation (if the conjugation table is hidden, click on the purple box 'Conjugation of е́сть (class irreg-b imperfective transitive)' to open it).
Есть can mean "to eat" or "exist(s)", these two words have the same spelling. In the meaning of "exist" есть is most often used when indicating possession as in "у меня есть" - "I have" (lit. by me there is). Ест is the third person singular conjugation of "to eat" used with she/he/it
Есть can mean "to eat" or "to exist", these two infinitives have the same spelling
«Есть» 'to exist' cannot be an infinitive «to exist», infinitive is «быть». «Есть» is a personal form (originally it was a 3rd person singular form, but it has since replaced all the other forms; although sometimes «суть» is used in very formal texts, and «есмь» in even more rare contexts for extra emphasis).
So, the verb with the infinitive есть 'to eat' is declined:
- 1 sg: я ем 'I eat, I'm eating',
- 2 sg: ты ешь 'you eat' (informal),
- 3 sg: он(а) ест 'she eats',
- 1 pl: мы едим 'we eat',
- 2 pl: вы еди́те 'you eat (can be used to refer to a single person)',
- 3 pl: они едя́т 'they eat',
The verb with the infinitive быть 'to be, to exist' is declined:
- 1 sg: я есть 'I am, I exist' (extremely rarely я есмь),
- 2 sg: ты есть 'you are, you exist' (almost never ты еси, except fairy tales),
- 3 sg: он(а) есть '(s)he is, (s)he exists',
- 1 pl: мы есть 'we are, we exist',
- 2 pl: вы есть 'you are, you exist',
- 3 pl: они́ есть 'they are, they exist' (rare: они́ суть).
However, in present tense «есть» is more often dropped than used.
'The/a girl ate an/the apple' = «Девочка съела яблоко». You use a different verb (съесть, not есть) because she wasn't just eating the apple (which is the meaning of «есть»), she successfully finished eating the apple (for which we use a different verb, «съесть»).
If you want to say that a/the girl was eating an apple but didn't neccessarily finish eating it, you could say «Девочка ела яблоко» (with the same verb, «есть», as in «Девочка ест яблоко»).
We use different verbs for this because we have much less tenses. English has different tenses (ate vs. was eating), while Russian has different verbs (съела vs. ела). It might seem strange at first, but you'll get used to this eventually. :)