Learn Finnish: Poika ja tyttö ovat myös täällä. - The boy and the girl are also here.

The conjunction ja (and) joins poika and tyttö together to form a combined plural subject. Even though "boy" and "girl" are singular words on their own, together they act as a "they", so the verb must take the plural form ovat.

Does the word ja change the endings of the words it connects?

No! Basic coordinating conjunctions like ja just link words together without changing their grammatical case. Because they act as the subject of this sentence, both poika and tyttö stay in their basic, unaltered dictionary form.

I have also learned the word tässä for "here". Why does this sentence use täällä?

Both words mean "here", but they are used slightly differently in Finnish. Tässä usually means "right exactly here" (like pointing at a specific spot or referring to something you are touching). Täällä is a bit broader and means "in this general area", "in this room", or "around here".

Could I put myös at the beginning of the sentence?

Yes, but it slightly changes what you are emphasizing. If you say Myös poika ja tyttö ovat täällä, it means "Also the boy and the girl are here" (meaning: in addition to the other people who are here). In our current sentence, placing myös after the verb flows exactly like the English translation: "are also here".