Je prends un billet aller simple pour cette destination.

Because after je, French needs a conjugated verb, not the infinitive.

• prendre = to take / to buy
• je prends = I take / I’m taking

So:

• Je prends un billet... = I’m taking / buying a ticket...

The form prends is the 1st person singular form of prendre in the present tense.

In this kind of situation, je prends can often mean something like I’ll take in natural English.

At a ticket counter or when making a choice, French often uses the present tense where English might use:

• I’m taking
• I’ll take
• I’ll have

So Je prends un billet aller simple can naturally mean:

• I’ll take a one-way ticket.

It is not always a strict word-for-word I take.

Because billet is a masculine noun in French.

So it takes:

• un billet = a ticket

not:

• une billet

This is something you simply have to learn with the noun.

Aller simple means one-way.

In travel French:

• un billet aller simple = a one-way ticket
• un billet aller-retour = a round-trip / return ticket

This is a fixed travel expression. Literally, it may not map neatly onto English word by word, so it is best learned as a chunk:

• billet aller simple = one-way ticket

Because in French, descriptive elements often come after the noun.

So instead of saying something like one-way ticket, French says:

• billet aller simple
• literally something closer to ticket one-way

This is normal French word order for many noun phrases.

Also, aller simple behaves like a fixed label in travel vocabulary, so learners should treat un billet aller simple as one unit.

Because aller simple is not being introduced as a separate noun phrase with de. It is part of a fixed expression describing the type of ticket.

So French says:

• un billet aller simple
• un billet aller-retour

not:

• un billet d’aller simple

Think of aller simple here as a set travel expression attached directly to billet.

Pour cette destination means for this destination.

Here, pour shows what place the ticket is intended for.

So:

• un billet ... pour cette destination = a ticket ... for this destination

That said, in very natural French, people often simply name the place:

• Je prends un billet aller simple pour Lyon.

Using cette destination is grammatically fine, but it can sound a bit formal or abstract compared with naming the city or country directly.

It is understandable and grammatically fine, but in real life, French speakers would often say something slightly more natural or more polite, such as:

• Je voudrais un aller simple pour cette destination.
• Je vais prendre un billet aller simple pour Paris.
• Un aller simple pour Paris, s’il vous plaît.

So Je prends un billet aller simple... works, but in a ticket-buying situation, je voudrais is often more polite.

Because destination is a feminine singular noun.

So the demonstrative adjective must also be feminine singular:

• ce = masculine singular
• cette = feminine singular
• ces = plural

Since destination is feminine:

• cette destination = this destination

Yes, very often.

In travel contexts, French speakers commonly shorten the phrase:

• un aller simple = a one-way ticket
• un aller-retour = a round-trip ticket

So both are possible:

• Je prends un billet aller simple.
• Je prends un aller simple.

The shorter version is very common in everyday speech.

Yes, simple normally means simple, but in the fixed expression aller simple, it means the trip is only one way, not a return trip.

So you should not translate it word for word as simple going. Instead, learn the whole phrase as:

• aller simple = one-way
• aller-retour = round-trip / return

This is a case where the full expression matters more than the literal meanings of each word.

A helpful approximate pronunciation is:

Zhuh prahn uh(n) bee-yay ah-lay sah(n)pl poor set dest-ee-na-syon.

A few notes:

• Je sounds like zhuh
• prends has a nasal vowel; the ds are not fully pronounced
• billet sounds like bee-yay
• aller sounds like ah-lay
• simple in this phrase is pronounced roughly sah(n)pl
• cette sounds like set

If you want to sound natural, focus especially on:

• je prends
• billet
• aller simple

Those are the parts learners often mispronounce first.