onnanoko ha akai bousi wo kabutte imasu.

It is read:

おんなのこ は あかい ぼうし を かぶって います

A few useful pronunciation notes:

• 女の子 = おんなのこ
• 帽子 = ぼうし
• The particle を is usually pronounced o in modern Japanese.

Also, in normal Japanese writing, this would usually appear without spaces:

女の子は赤い帽子をかぶっています。

When は is used as the topic particle, it is pronounced wa, not ha.

So in this sentence:

• 女の子は = As for the girl...

This is just a special pronunciation for the particle は. When は is part of a normal word, it is still pronounced ha.

Both can be correct, but the nuance is different.

• 女の子は赤い帽子をかぶっています。
  = As for the girl, she is wearing a red hat.
  This makes the girl the topic.

• 女の子が赤い帽子をかぶっています。
  = The girl is wearing a red hat or It is the girl who is wearing a red hat.
  This puts more focus on who is doing the action.

For a simple neutral sentence, especially when the girl is already the topic of conversation, は is very natural.

In 女の子 (おんなのこ), the の is part of the fixed word for girl.

Historically, it is built from:

• 女 = woman/female
• 子 = child

So literally it is something like female child, but in modern Japanese you should usually just learn 女の子 as one vocabulary item meaning girl.

It is not being used here as a normal possessive like A's B.

赤い is an い-adjective, and い-adjectives can directly modify a noun.

So:

• 赤い = red
• 帽子 = hat
• 赤い帽子 = red hat

You do not need な here, because 赤い is not a な-adjective.

So these are correct and incorrect:

• 赤い帽子 = correct
• 赤な帽子 = incorrect

The particle を marks the direct object of the verb.

Here, the verb is かぶる, which means to wear on the head or to put on one's head. The thing being worn is 帽子, so it takes を:

• 帽子をかぶる = to wear a hat

Even though English says wear a hat, Japanese still treats hat as the direct object of the verb, so を is used.

かぶっています comes from the dictionary form かぶる.

Breakdown:

• かぶる = to put on / wear on the head
• かぶって = te-form of かぶる
• います = polite form of いる

Together, かぶっています is the 〜ている form.

In many cases, 〜ている means is doing, but with clothing verbs it often describes the current state resulting from the action. So here it means:

• is wearing

not necessarily is in the middle of putting on

That is why かぶっています is natural for describing what someone has on.

かぶります is the polite non-past form of かぶる. Depending on context, it can mean things like:

• will put on
• puts on
• wears as a habit

But when you are describing what someone is wearing right now, Japanese usually prefers 〜ています:

• 帽子をかぶっています = is wearing a hat

So in a picture description or a statement about someone’s current appearance, かぶっています is the more natural choice.

Japanese often uses different verbs depending on what part of the body the item goes on.

Common examples:

• かぶる = wear on the head
  
  • 帽子をかぶる = wear a hat
• 着る（きる） = wear on the upper body
  
  • シャツを着る = wear a shirt
• はく = wear on the lower body or feet
  
  • ズボンをはく = wear pants
  • 靴をはく = wear shoes
• かける = wear on the face
  
  • めがねをかける = wear glasses
• する = wear accessories
  
  • ネクタイをする = wear a tie

So かぶる is used because a hat goes on the head.

Normally, Japanese is written without spaces:

女の子は赤い帽子をかぶっています。

The spaces in your example are just a learning aid to make the parts easier to see:

• 女の子
• は
• 赤い
• 帽子
• を
• かぶって
• います

As your reading improves, it is good to get used to seeing the sentence written without spaces, because that is how it appears in real Japanese.