Definitivno ćemo uštedeti.

Serbian future here is made with:

• ćemo = the future auxiliary for we
• uštedeti = the main verb in the infinitive

So the pattern is:

• ću / ćeš / će / ćemo / ćete / će + infinitive

Examples:

• Ja ću doći = I will come
• Mi ćemo uštedeti = We will save

So ćemo uštedeti literally works like we will save.

Because ću, ćeš, će, ćemo, ćete, će are clitics in Serbian. Clitics usually go in the second position of the sentence or clause.

So in:

• Definitivno ćemo uštedeti

the first element is definitivno, and the clitic ćemo comes right after it.

This is very normal Serbian word order.

Serbian often leaves out subject pronouns when they are already clear from the verb form.

Here, ćemo already tells you the subject is we.

So:

• Definitivno ćemo uštedeti = normal, natural
• Mi ćemo definitivno uštedeti = also correct, but mi adds emphasis or contrast

You would use mi if you want something like:

• We will definitely save, but they won’t.

Ćemo is the 1st person plural form of the future auxiliary, related to the verb hteti.

Its future forms are:

• ću = I will
• ćeš = you will
• će = he/she/it will
• ćemo = we will
• ćete = you will
• će = they will

So ćemo by itself already carries the idea of we will.

This is about aspect, which is very important in Serbian.

• štedeti = imperfective
  meaning something like to save / to be saving / to economize as an ongoing or repeated action

• uštedeti = perfective
  meaning to save up / to manage to save / to achieve the result of saving

In this sentence, the speaker is talking about a result:

• We will definitely save
  meaning we will succeed in saving or we will end up with savings

That is why uštedeti fits well.

Yes. Serbian can also form the future as one written word when the main verb comes before the auxiliary.

So you can have:

• Uštedećemo = We will save

This is related to:

• ćemo uštedeti

In your sentence, though, the neutral and natural form is:

• Definitivno ćemo uštedeti

If you rearrange the sentence, you may get:

• Uštedećemo, definitivno

That is possible, but it sounds more marked or emphatic.

Yes. In everyday Serbian, definitivno very often means:

• definitely
• certainly
• for sure

Even though it looks like English definitively, in normal usage it usually matches definitely better.

Another common Serbian word with a similar meaning is:

• sigurno = surely / definitely / certainly

Yes. Serbian word order is fairly flexible, but changing it changes the emphasis.

For example:

• Definitivno ćemo uštedeti = neutral, natural
• Mi ćemo definitivno uštedeti = emphasizes we
• Uštedećemo definitivno = stronger emphasis on the action/result

The basic meaning stays similar, but the focus shifts.

That is a dialect and standard-language difference.

• uštedeti = Ekavian
• uštedjeti = Ijekavian

Both mean the same thing.

If you are learning Serbian as used in most of Serbia, you will very often see uštedeti.
If you read Bosnian, Montenegrin, or some Serbian varieties, you may see uštedjeti.

A rough guide:

• ć is a soft sound, somewhat between English t and ch
• š is like English sh

Approximate pronunciation:

• ćemo ≈ tyeh-mo with a very soft first consonant
• uštedeti ≈ oo-SHTEH-deh-tee

Also remember that Serbian vowels are usually clear and simple:

• e is always like e in met, not like English silent e patterns