Cuando voy a la playa, siempre llevo crema solar.

Because this sentence describes a habit or something that happens regularly.

• Cuando voy a la playa... = When/Whenever I go to the beach...
• Spanish normally uses the present indicative after cuando for repeated, usual actions.

You would use cuando vaya if you were talking about a future occasion:

• Cuando vaya a la playa, llevaré crema solar.
  When I go to the beach, I’ll take sunscreen.

So in your sentence, voy is correct because it means whenever I go.

In Spanish, the simple present is very often used for habitual actions, just like in English:

• Cuando voy a la playa...
• Siempre llevo crema solar.

This is the normal way to say I usually do this or I always do this.

Spanish does not need a special tense for habitual actions here.

Spanish often omits subject pronouns because the verb ending already shows who the subject is.

• voy = I go
• llevo = I take / I carry

So yo is unnecessary unless you want emphasis or contrast:

• Yo siempre llevo crema solar, pero mi hermano nunca lleva.
  I always take sunscreen, but my brother never does.

In a normal sentence, leaving out yo sounds more natural.

Because ir a means to go to, and playa is a feminine noun:

• la playa = the beach
• a + la playa = to the beach

So:

• voy a la playa = I go to the beach

Unlike a + el, which contracts to al, a + la does not contract.

• a + el cine = al cine
• a + la playa = a la playa

Spanish often uses the definite article with places people go to in a general, familiar way.

So these are natural:

• ir a la playa
• ir al cine
• ir a la piscina

English often leaves out the in similar expressions, but Spanish usually keeps it. So a la playa is the normal choice.

Here, llevo means I take / I bring with me / I carry.

The verb llevar is very broad. It can mean:

• to carry
• to take
• to wear
• to have on
• to bring along

In this sentence, the idea is that when you go to the beach, you take sunscreen with you.

Because llevar usually means to take something away from where you are / to carry something with you, while traer usually means to bring something toward the speaker or destination.

So if you are talking about what you take with you to the beach, llevar is the natural choice:

• Llevo crema solar a la playa.

Very roughly:

• llevar = take
• traer = bring

English often uses bring more broadly than Spanish does, so this is a common point of confusion.

Because Spanish often leaves out the article when talking about an item in a general, non-specific way, especially after verbs like llevar, comprar, tener, or usar.

So:

• llevo crema solar = I take sunscreen

This means sunscreen in general, not a specific bottle.

If you mean a particular one, the article can appear:

• Llevo la crema solar que compré ayer.
  I’m taking the sunscreen I bought yesterday.

Yes, crema solar is very common and natural in Spain.

You may also hear:

• protector solar
• crema de protección solar

In some Latin American countries, bloqueador or bloqueador solar is also common, but for Spain, crema solar is a very good choice.

That is a very normal position for adverbs like siempre, nunca, and a menudo.

• Siempre llevo crema solar. = I always take sunscreen.

This placement sounds natural and neutral.

You may also hear:

• Llevo siempre crema solar.

That is possible, but siempre llevo is the more standard and common order for a basic sentence like this.

It can be understood as either, but in this sentence it has the sense of whenever, because the action is repeated.

• Cuando voy a la playa, siempre llevo crema solar.

This means something like:

• Whenever I go to the beach, I always take sunscreen.

English often uses when for this kind of habitual meaning too, so both are possible depending on the translation style.

Yes, but it changes the meaning.

• Llevo crema solar = I take/bring sunscreen with me
• Uso crema solar = I use sunscreen
• Me pongo crema solar = I put sunscreen on / I apply sunscreen

So llevo focuses on bringing it along, not necessarily applying it.

In most of Spain, ll is pronounced like the y sound in English yes.

So llevo sounds roughly like:

• YEH-bo or YEH-vo

Also, in Spanish, b and v are pronounced very similarly, so the v in llevo does not sound like a strong English v.

A rough pronunciation of the whole sentence is:

• KWAHN-doh BOY ah lah PLA-yah, see-EM-preh YEH-bo KREH-mah soh-LAR.

That is only an approximation, but it helps as a starting point.