Nisam spreman sada, ali ću uskoro biti.

Nisam means I am not.

It is the negative present-tense form of biti (to be) for ja (I):

• sam = I am
• nisam = I am not

In Serbian, this negative form is not written as two separate words here. You learn it as a single form:

• ja sam = I am
• ja nisam = I am not

So Nisam spreman sada literally means I am not ready now.

Yes. Spreman is an adjective, and it agrees with the speaker in gender and number.

Here, spreman is the masculine singular form, so it suggests the speaker is male.

Other forms would be:

• spremna = feminine singular
• spremni = masculine personal plural or mixed group
• spremne = feminine plural

So a woman would say:

• Nisam spremna sada, ali ću uskoro biti.

English does not change ready this way, but Serbian adjectives do.

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

In nisam, the ending already tells you it is I.
In ću, it also means I will.

So ja is not necessary:

• Ja nisam spreman sada... = correct
• Nisam spreman sada... = also correct, and more natural in many contexts

Serbian does this much more often than English.

Ću is the short form of the verb hteti (to want) used as a future-tense auxiliary. In modern Serbian, it helps form the future tense, like will in English.

So:

• ću biti = I will be

In this sentence:

• ali ću uskoro biti = but I will be soon

The full form related to ću is hoću, but in normal future-tense sentences the short form ću is what you usually see.

Because Serbian word order is flexible, and short forms like ću tend to appear near the beginning of the clause.

So the future part is arranged like this:

• ću = will
• uskoro = soon
• biti = be

This gives:

• ću uskoro biti

That is a natural Serbian order.

A very literal word-for-word mapping would look odd in English, but the meaning is simply:

• I will be soon

Because Serbian, like English, often leaves out repeated information when it is obvious.

The full version could be:

• Nisam spreman sada, ali ću uskoro biti spreman.

But repeating spreman is unnecessary, so Serbian usually shortens it to:

• Nisam spreman sada, ali ću uskoro biti.

This is just like English:

• I’m not ready now, but I will be soon.

You do not need to repeat ready there either.

Yes. Serbian word order is more flexible than English word order.

Both of these are natural:

• Nisam spreman sada.
• Nisam sada spreman.

They mean the same basic thing: I’m not ready now.

The difference is usually one of emphasis:

• Nisam spreman sada can sound like the focus is on ready
• Nisam sada spreman can sound like the focus is on now

These are small differences, not completely different meanings.

Both mean now.

• sada = full form
• sad = shorter, very common spoken form

In many situations they are interchangeable:

• Nisam spreman sada.
• Nisam spreman sad.

Both are natural. Sad is a bit more conversational, while sada can sound slightly more neutral or careful.

Uskoro means soon, so it talks about time.

• uskoro = soon, before long

Brzo usually means quickly/fast, so it talks more about speed.

Compare:

• Doći ću uskoro. = I will come soon.
• Doći ću brzo. = I will come quickly / fast.

In your sentence, uskoro is the right choice because the idea is not now, but soon.

Normally, no. Ću is a short clitic form, and clitics usually do not stand at the very beginning of a clause.

That is why ali ću uskoro biti works: ali comes first, and ću follows it.

If you wanted to say that idea by itself, you would more naturally say something like:

• Uskoro ću biti spreman.
• Ja ću uskoro biti spreman.
• Biću uskoro spreman.

All of these are natural ways to express I will be ready soon.

Because ali means but, and in Serbian it normally introduces a new clause, so it is usually preceded by a comma.

So:

• Nisam spreman sada, ali ću uskoro biti.

This works just like English:

• I’m not ready now, but I will be soon.

The comma helps separate the two contrasting ideas:

• not ready now
• will be soon