sich etwas zulegen
to get for oneself
To buy or acquire something new for oneself, usually a larger or more significant item.
Wir wollen uns nächstes Jahr ein neues Auto zulegen.
We want to get ourselves a new car next year.

to acquire / to gain
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to get for oneself
To buy or acquire something new for oneself, usually a larger or more significant item.
Wir wollen uns nächstes Jahr ein neues Auto zulegen.
We want to get ourselves a new car next year.

'Zulegen' (to acquire / to gain) is a separable verb. The prefix 'zu' comes at the end in a main clause, but stays attached to the verb in a subordinate clause. With 'zu' + infinitive, 'zu' comes between the prefix and the verb.
Er legt sich ein neues Handy zu.
He is getting himself a new phone.
The verb consists of the prefix 'zu-' and the verb 'legen' (to lay/put). The prefix 'zu-' often expresses adding or increasing something, which can be seen in all meanings.
Sie hat im letzten Jahr an Erfahrung deutlich zugelegt.
She has gained significantly in experience over the past year.
In the meaning 'sich etwas anschaffen' (to acquire something for oneself), the verb is used reflexively with the dative and an accusative object: someone (nominative) gets themselves (dative) something (accusative).
Ich habe mir endlich einen neuen Laptop zugelegt.
I finally got myself a new laptop.
When it is about an increase, 'zulegen' (to gain/increase) is often used with the preposition 'an' + dative, for example 'an Gewicht zulegen' (to gain weight) or 'an Tempo zulegen' (to pick up speed).
Der Läufer hat auf der letzten Runde an Geschwindigkeit zugelegt.
The runner picked up speed on the final lap.
In the meaning 'schneller werden' (to get faster), 'zulegen' is often used as a command or statement and can stand without an object.
Leg mal zu, sonst verpassen wir den Bus!
Speed up, or we'll miss the bus!
to buy
'kaufen' and 'sich etwas zulegen' can both mean that you get something in exchange for money. 'kaufen' is the normal and direct word for almost all purchases, whether bread, clothes, or a car. 'sich etwas zulegen' is more suitable when you acquire something new that is somewhat more important, larger, or useful for a longer time, for example a bicycle, a phone, or a sofa. Learners may confuse the words because money is often paid in both cases. As a practical rule: for the individual act of buying, you usually use 'kaufen'. If you want to emphasize that you now own or are acquiring something new, 'sich zulegen' sounds more natural.

Nach dem Umzug legt sich Max endlich einen eigenen Schreibtisch zu.
After the move, Max is finally getting himself his own desk.
Laura kauft heute am Telefon zwei Tickets für das Konzert.
Laura is buying two tickets for the concert over the phone today.