How do you say the word buy in spanish?

The infinitive form of 'to buy' would be: comprar

Conjugating the verb is done like this:

I buy: yo compro

You (familiar) buy: tú compras

You (formal) buy: usted compra

he/she buys: él compra/ella compra

we buy: nosotros compramos

they buy: ellos/ellas compran

you (plural) buy: ustedes compran