Kad ne znam čime početi, najprije napravim kavu, sjednem i napišem kratki plan.

Here kad means when, and in this kind of sentence it often has the broader sense of whenever.

Kad and kada mean the same thing. The difference is mostly style:

• kad = shorter, very common, natural in everyday Croatian
• kada = a bit fuller and sometimes slightly more formal

So Kad ne znam... is completely normal.

Because Kad ne znam čime početi is a subordinate clause placed before the main clause.

Croatian normally separates an introductory clause like this with a comma:

Kad ne znam čime početi, ...

That comma works much like English punctuation in sentences such as When I do not know where to start, ...

Čime is the instrumental form of što.

A rough literal idea is with what. So:

• čime početi = what to start with

This is a very natural Croatian pattern when talking about the first thing, tool, or step you begin with.

After znam / ne znam, Croatian often uses a question word + infinitive structure:

• Ne znam što reći.
• Ne znam kamo ići.
• Ne znam kako početi.
• Ne znam čime početi.

So ne znam čime početi is completely normal.

A very common alternative is:

Ne znam čime da počnem.

Both are natural. The version with the infinitive is a bit more compact.

Because Croatian usually does not need subject pronouns when the verb ending already shows the person.

Here:

• znam
• napravim
• sjednem
• napišem

all clearly show I.

You would add ja only for emphasis or contrast, for example:

Ja napravim kavu, a on ode van.

In Croatian, this is normal. The sentence describes what happens each time that situation occurs.

With kad meaning when/whenever, Croatian can use present-tense forms to describe a repeated pattern:

• first I make coffee
• then I sit down
• then I write a short plan

So this is not strange at all. It is a standard way to express a habitual sequence.

Because the speaker presents each action as a whole completed step in a sequence.

• napravim = I make it
• sjednem = I sit down
• napišem = I write it out / finish writing it

This gives the sentence a clear step-by-step feeling: one complete action after another.

If you used imperfective verbs, the focus would be more on the process or ongoing activity, not on each finished step.

Znati is imperfective here, because it describes a state: not knowing.

So the sentence combines:

• a state: ne znam
• followed by completed actions: napravim, sjednem, napišem

That contrast is very natural in Croatian: first the situation exists, then the speaker responds with a sequence of actions.

Najprije means first or first of all.

Yes, prvo is also possible and very common. The difference is small:

• najprije = slightly more formal or careful
• prvo = very common, everyday

So both of these work:

• najprije napravim kavu
• prvo napravim kavu

Yes, very natural.

Napraviti kavu is a common way to say make coffee.

Other possible verbs are:

• skuhati kavu = to brew / cook coffee
• pripremiti kavu = to prepare coffee

So napravim kavu is idiomatic and everyday.

Because kratki agrees with plan in gender, number, and case.

Here plan is:

• masculine
• singular
• the direct object of napišem

So the adjective must match it:

• kratki plan

Also, plan is masculine inanimate, so in the accusative singular it looks the same as in the nominative.

Yes. Od čega početi is also very common.

Both are natural, but the nuance is slightly different:

• čime početi = what to start with
• od čega početi = where / from what to begin

In many real situations, they are very close in meaning.

Not completely. Croatian word order is fairly flexible, and changing it can shift emphasis.

For example, najprije can move:

• Kad ne znam čime početi, najprije napravim kavu...
• Kad ne znam čime početi, napravim najprije kavu...

The first version sounds more natural and neutral.

So the original word order is very good, but Croatian often allows alternatives when the speaker wants a different rhythm or emphasis.