Learn Turkish: Senin alman gereken mendil indirimde. - The wipe you must buy is on sale.

This is the standard formula for creating relative clauses of necessity. It combines a verbal noun with a possessive ending (al-ma-n, literally "your buying") and the word gereken, which is the relative clause form of gerek ("which is necessary"). Literally, it means "the wipe which necessitates your buying," which translates to "the wipe you must buy" in natural English.

Why does the sentence start with the genitive Senin instead of Sen?

Because the action of buying is expressed as a noun (alman, "your buying"), its subject acts like an owner and must take the genitive case (Senin). You could actually drop Senin completely since the -n at the end of alman already tells us it is you who must buy it. Including it here just adds emphasis.

Doesn't mendil usually mean "tissue" or "handkerchief"?

Yes, it traditionally means handkerchief or tissue. However, it is also the standard word for a "wipe," most commonly seen in the phrase ıslak mendil (wet wipe). Given the context of buying something on sale at a modern store, "wipe" or "tissue" are both perfectly natural translations.

Why is the locative case -de added to indirim to make indirimde?