asita ha tomodati to issyo ni ani no tiimu wo ouensi ni ikimasu.

Because 明日は makes tomorrow the topic of the sentence.

So the feeling is something like:

• As for tomorrow, I’m going...

A time word like 明日 can also appear with no particle at all:

• 明日、友達と一緒に…行きます。

That is also natural. Adding は gives a little more framing, and sometimes a slight contrastive feeling, like tomorrow, at least or as for tomorrow. In this sentence, it most likely just sets the topic naturally.

Japanese often leaves out the subject when it is obvious from context.

In English, you usually need to say I, but in Japanese, if it is clear that the speaker is talking about their own plans, 私は is often unnecessary.

So this sentence naturally implies:

• I am going tomorrow with a friend to cheer for my older brother’s team.

You could say 私は明日は… or 私は明日…, but that often sounds more explicit than necessary.

They overlap, but they are not exactly the same.

• 友達と = with a friend / together with a friend
  
  • Here と marks the companion.
• 一緒に = together

So:

• 友達と行きます = I’m going with a friend.
• 友達と一緒に行きます = I’m going together with a friend.

Using both is very common and sounds natural. 一緒に adds a little emphasis to the idea of being together.

の links nouns, and here it works like ’s in English.

• 兄 = older brother
• 兄のチーム = older brother’s team

So 兄のチーム means my older brother’s team.

Japanese often uses noun + の to show possession or relationship:

• 先生の本 = the teacher’s book
• 日本の文化 = Japanese culture
• 友達の家 = a friend’s house

When talking about your own family to someone else, Japanese often uses the plain family terms:

• 兄 = my older brother
• 父 = my father
• 母 = my mother

Forms like お兄さん, お父さん, and お母さん are commonly used:

• when talking about someone else’s family
• when addressing your own family directly

So in this sentence, 兄のチーム is the normal way to say my older brother’s team.

Because 応援する takes a direct object in Japanese.

• チームを応援する = to support / cheer for the team

In English, we often say cheer for with a preposition, but Japanese uses を here.

More examples:

• 選手を応援する = cheer for a player
• 友達を応援する = support a friend

So even though English uses for, Japanese treats the thing being supported as the direct object.

This is a very common pattern:

• verb stem + に + 行く = go to do ...
• verb stem + に + 来る = come to do ...

Here:

• 応援する = to cheer/support
• verb stem = 応援し
• 応援しに行きます = go to cheer / go in order to cheer

So the structure is:

• [purpose] + に + 行きます

Other examples:

• 食べに行きます = go to eat
• 見に行きます = go to see
• 勉強しに来ました = came to study

Because in this pattern, Japanese uses the verb stem, not the dictionary form.

So:

• 応援する → 応援し
• 応援しに行く = go to cheer

You do the same with many verbs:

• 食べる → 食べ → 食べに行く
• 見る → 見 → 見に行く
• 話す → 話し → 話しに行く

So 応援するに行く is not the normal form. The correct pattern is 応援しに行く.

Because Japanese sentences normally put the main verb at the end.

The main action here is 行きます = go. Everything before it gives extra information:

• 明日は = as for tomorrow
• 友達と一緒に = with a friend / together with a friend
• 兄のチームを = my older brother’s team
• 応援しに = to cheer for

A rough structural breakdown is:

• Tomorrow, with a friend, my older brother’s team, to cheer for, I will go.

That sounds strange in English, but it is normal in Japanese. Japanese word order is somewhat flexible because particles show each word’s role, but the final verb is still a very strong pattern.

It is polite because it ends with 行きます.

• 行きます = polite
• 行く = casual/plain

So the casual version would be:

• 明日は友達と一緒に兄のチームを応援しに行く。

Also, notice that 応援し is not a separate politeness choice here. It is just the verb stem used in the purpose + に + 行く pattern. The politeness of the whole sentence is shown mainly by the final verb 行きます.