Večeras ću njoj poslati poruku.

Because ću is a clitic. In Serbian, clitics usually go in the second position of the clause, after the first stressed word or phrase.

So here:

• Večeras = first element
• ću = goes right after it

That is why the sentence is:

Večeras ću njoj poslati poruku.

not normally:

Večeras poslati ću njoj poruku.

If the sentence started with ja, then you would get:

Ja ću njoj poslati poruku večeras.

The future is formed with:

• ću = the 1st person singular future auxiliary
• poslati = the infinitive to send

So the structure is:

ću + infinitive

Here it means I will send.

A very literal breakdown is:

• Večeras = tonight
• ću = will
• njoj = to her
• poslati = send
• poruku = a message

Because Serbian often drops subject pronouns when they are already clear from the verb form.

Here, ću already tells you the subject is I.

So:

• Večeras ću njoj poslati poruku. = Tonight I will send her a message.

You can add ja, but usually only for emphasis or contrast:

• Ja ću njoj poslati poruku. = I will send her a message.
• Ja ću njoj poslati poruku, ne ti. = I will send her a message, not you.

Both mean to her, but they are used differently.

• joj = unstressed clitic form
• njoj = stressed/full form

The more neutral sentence would usually be:

Večeras ću joj poslati poruku.

Using njoj often adds emphasis or contrast, as if you are stressing to her in particular.

So:

• Večeras ću joj poslati poruku. = neutral
• Večeras ću njoj poslati poruku. = more emphatic, something like Tonight I’ll send a message to her

Because the verb poslati takes the recipient in the dative case directly.

So Serbian says:

• poslati nekome poruku = to send someone a message

There is no preposition like to in Serbian here. The dative ending itself carries that meaning.

So:

• njoj = to her
• not za nju or anything like that in this sentence

Because poruku is the accusative singular form of poruka.

• poruka = nominative, the basic dictionary form
• poruku = accusative, used for the direct object

Since message is the thing being sent, it has to be in the accusative:

• poslati poruku = to send a message

This is a question of aspect, which is very important in Serbian.

• poslati = perfective = to send, as a completed single action
• slati = imperfective = to be sending / to send repeatedly or habitually

In this sentence, the speaker means one completed action in the future, so poslati is the natural choice.

So:

• Večeras ću njoj poslati poruku. = Tonight I’ll send her a message.
• Večeras ću joj slati poruke. = Tonight I’ll be sending her messages or I’ll send her messages repeatedly

Yes. Serbian word order is fairly flexible, and changing it often changes emphasis, not the core meaning.

Possible versions include:

• Večeras ću joj poslati poruku. = neutral
• Njoj ću večeras poslati poruku. = emphasis on her
• Poruku ću joj poslati večeras. = emphasis on the message
• Večeras ću njoj poslati poruku. = emphasis on her

What usually stays important is the placement of clitics like ću and joj, which still follow clitic-order rules.

Yes. In Serbian, especially in everyday speech, future can also be expressed with:

ću + da + present tense

So this is also natural:

Večeras ću da joj pošaljem poruku.

That version means the same thing as:

Večeras ću joj poslati poruku.

The sentence you were given uses the infinitive, which is very standard and common in grammar explanations.

Večeras means tonight or this evening, depending on context.

It is an adverb, so you do not need a preposition with it in this sentence.

Examples:

• Večeras radim. = I’m working tonight.
• Večeras ću njoj poslati poruku. = Tonight I’ll send her a message.

So večeras sets the time for the whole action.