Я спросила официанта, какой соус лучше для рыбы.

Because Russian past-tense verbs show gender in the singular.

• Я спросила = I asked said by a woman
• Я спросил = I asked said by a man

So the sentence tells you the speaker is female.

This is a question about aspect.

• спросила is perfective: one completed action
  
  • I asked
• спрашивала is imperfective: the process, repetition, or background action
  
  • I was asking / I used to ask / I asked (with focus on the process, not the result)

Here, спросила fits because the speaker asked once and completed the action.

Because официанта is the accusative singular form.

The verb спросить usually takes the person asked as a direct object:

• спросить официанта = to ask the waiter

For animate masculine nouns, the accusative singular looks like the genitive singular:

• nominative: официант
• accusative: официанта

That is why you see -а here.

Yes, you can.

Both are natural:

• Я спросила официанта...
• Я спросила у официанта...

The version without у treats официанта as the direct object: I asked the waiter.
The version with у emphasizes getting information from him: I asked the waiter / I asked from the waiter.

In everyday Russian, both are common.

Because какой often means what kind of / which when asking about type, quality, or suitability.

Here the speaker is asking which sauce would be better for fish in general, so какой is natural.

• какой соус лучше для рыбы = which sauce is better for fish?

который is more often used when choosing from a specific known set:

• Который соус вы хотите — грибной или сырный?
  = Which sauce do you want — mushroom or cheese?

So какой works well here because the focus is on the kind of sauce that suits fish best.

Because какой must agree with соус in gender, number, and case.

• соус is masculine singular
• so the correct form is какой

Compare:

• какой соус — masculine
• какая рыба — feminine
• какое блюдо — neuter

Because лучше is the form normally used in this kind of sentence.

• лучше = better
• лучший = best as an adjective

Russian often uses the comparative лучше where English might naturally say best, especially in choice questions:

• Какой соус лучше для рыбы?
  literally: Which sauce is better for fish?
  natural meaning: Which sauce is best for fish?

Using лучший would require a different structure, and it is usually less natural here.

Because Russian usually omits the present-tense form of быть (to be).

So:

• какой соус лучше для рыбы
  literally = which sauce better for fish

But in natural English, it means:

• which sauce is better for fish

This is completely normal in Russian.

Для takes the genitive case, so рыба becomes рыбы.

• dictionary form: рыба
• after для: для рыбы

So:

• для рыбы = for fish

This is grammatically correct. In food contexts, Russian also often uses к рыбе to mean with fish / to go with fish, but для рыбы is still understandable and correct.

Because какой соус лучше для рыбы is a subordinate clause.

The main clause is:

• Я спросила официанта = I asked the waiter

Then comes the embedded question:

• какой соус лучше для рыбы = which sauce is better for fish

Russian normally separates this kind of subordinate clause with a comma.

It is an indirect question, not a direct question.

Compare:

• direct question: Какой соус лучше для рыбы?
• indirect question: Я спросила официанта, какой соус лучше для рыбы.

In the full sentence, the whole thing is a statement: I asked the waiter...
That is why there is a period, not a question mark.

Yes, often you can, if the context makes the subject clear.

• Я спросила официанта, какой соус лучше для рыбы.
• Спросила официанта, какой соус лучше для рыбы.

Russian often drops subject pronouns when they are understood from context.
However, in the past tense, the verb shows gender but not clearly person, so omitting Я depends more on context than in the present tense.

Yes. Russian word order is fairly flexible.

The original sentence is neutral and natural:

• Я спросила официанта, какой соус лучше для рыбы.

You could also say:

• Я спросила официанта, какой соус для рыбы лучше.

The meaning stays almost the same, but the emphasis shifts slightly.
For a learner, the original order is a very good default.