Questo shampoo fa molta schiuma, quindi devo risciacquare bene i capelli.

Because shampoo is treated as a masculine singular noun in Italian, so it takes questo.

• questo = this (masculine singular)
• questa = this (feminine singular)

So:

• questo shampoo
• questa crema

Even though shampoo is a borrowed word from English, in Italian it is normally masculine.

Yes, literally it means makes a lot of foam/lather, and that is a very natural way to express this idea in Italian.

Italian often uses fare + noun where English might use a different verb. Here:

• fare schiuma = to lather / to foam up
• fare molta schiuma = to produce a lot of lather

So although the literal wording is different from English, it is the normal idiomatic phrasing in Italian.

Because schiuma here is being treated as an uncountable noun, like foam or lather in English.

So Italian says:

• molta schiuma = a lot of foam/lather

not usually:

• molte schiume

Also, molta agrees with schiuma, which is feminine singular:

• molto = masculine singular
• molta = feminine singular
• molti = masculine plural
• molte = feminine plural

Yes, produce molta schiuma is possible and perfectly understandable, but fa molta schiuma sounds more everyday and natural in normal speech.

Compare:

• Questo shampoo fa molta schiuma. = very common, conversational
• Questo shampoo produce molta schiuma. = a bit more formal or technical

So if you want the most natural spoken Italian, fa molta schiuma is the better choice.

Here quindi means so, therefore, or as a result.

In this sentence:

• Questo shampoo fa molta schiuma, quindi devo risciacquare bene i capelli.
• This shampoo lathers a lot, so I have to rinse my hair well/thoroughly.

It commonly connects two clauses like this. Its position is fairly flexible, but the version in your sentence is very natural.

For example:

• Fa molta schiuma, quindi devo risciacquare bene i capelli.
• Questo shampoo quindi fa molta schiuma.
  This is possible, but the meaning/focus changes slightly.

Because Italian normally drops subject pronouns when they are not needed.

• devo already means I must / I have to
• so io devo is usually unnecessary

Italian verb endings often make the subject clear:

• devo = I have to
• devi = you have to
• deve = he/she/it has to

You can add io for emphasis or contrast:

• Io devo risciacquare bene i capelli, tu no.
• I have to rinse my hair well, you don’t.

They are very close in meaning. Both can mean to rinse.

• risciacquare often feels a bit more explicit as to rinse out / rinse thoroughly
• sciacquare is also common and perfectly correct

In many everyday contexts, they can be interchangeable:

• Devo risciacquare i capelli.
• Devo sciacquare i capelli.

With shampoo, risciacquare is especially common because the idea is rinsing the product out.

Bene here means well, but in this context it often comes across more naturally in English as thoroughly.

So:

• risciacquare bene i capelli = to rinse the hair well / thoroughly

It suggests you need to remove the shampoo completely, especially because it makes a lot of lather.

This is very common in Italian: bene can mean not just in a good way, but also properly or thoroughly, depending on context.

Italian often uses the definite article with body parts where English uses a possessive.

So instead of saying:

• i miei capelli = my hair

Italian very often just says:

• i capelli

because it is already clear whose hair is meant.

In this sentence, the speaker says devo (I have to), so it is obvious they mean their own hair.

That said, i miei capelli is also possible if you want to be more explicit or contrastive:

• Devo risciacquare bene i miei capelli, non i tuoi.

Italian usually uses capelli in the plural when talking about the hair on someone’s head.

So:

• i capelli = hair / the hairs on the head

English usually treats hair as an uncountable singular noun, but Italian commonly uses the plural count noun.

Examples:

• Ho i capelli lunghi. = I have long hair.
• Mi lavo i capelli. = I wash my hair.

This is one of those places where Italian and English organize the idea differently.

Yes, absolutely. It is grammatically correct.

However, i capelli is more natural if there is no need to stress ownership. Italian prefers the simpler article when the possessor is obvious.

So:

• devo risciacquare bene i capelli = more natural in most situations
• devo risciacquare bene i miei capelli = more explicit, sometimes more emphatic

Yes, shampoo is normally masculine:

• lo shampoo = the shampoo
• questo shampoo = this shampoo

As for the plural, many borrowed words in Italian are invariable, so the form stays the same:

• uno shampoo
• due shampoo

The article or context shows whether it is singular or plural.

Usually yes. Capelli refers to the hair on the head.

Italian distinguishes this from peli, which means body hair or fur.

So:

• capelli = head hair
• peli = body hair / animal fur

That is why risciacquare bene i capelli clearly means rinsing the hair on your head after using shampoo.