Learn Spanish (Latin America): Yo soy el primer estudiante.

In Spanish, the word for "first" is primero. However, when it comes directly before a masculine singular noun (like estudiante in this sentence), it drops the final -o and becomes primer.

Why does primer come before the noun? I thought Spanish adjectives usually go after.

You are right that most descriptive adjectives (like colors or sizes) go after the noun. But ordinal numbers, which show order like first, second, or third, are an exception! They almost always go before the noun, just like in English.

What if the speaker is female? How would the sentence change?

The noun estudiante stays exactly the same for both men and women, but the words around it must match the gender. A female student would say Yo soy la primera estudiante. The word primero only shrinks to primer for masculine nouns; for feminine nouns, it changes to primera.

Why do we use soy instead of estoy for "I am" here?

We use soy (from the verb ser) for things that describe identity, traits, or facts. Being "the first student" is treated as a defining fact or status in this context, rather than a temporary condition or location, so soy is the right choice.