Learn Russian: Она весёлая девочка.

In Russian, the present tense of the verb "to be" is skipped. So, Она весёлая девочка literally translates as "She cheerful girl" but means "She is a cheerful girl".

Why does the adjective весёлая end in -ая?

Russian adjectives change their endings to match the gender of the noun they describe. Since девочка is a feminine word, the adjective takes the feminine ending -ая. If you were describing a masculine word, like a boy, the ending would change.

Can I use девочка for a young woman?

No, девочка specifically means a female child or little girl. If you want to talk about a young woman or teenage girl, you should use the word девушка.

How do you pronounce the ё in весёлая?

The letter ё is pronounced like "yo" in "yoyo". It is a very helpful letter because whenever a Russian word has a ё, that syllable is always the stressed one! Keep in mind that native speakers often write it simply as е without the dots, but it keeps the "yo" sound.