Ils louent une petite chambre près du métro.

Louer primarily means “to rent” (i.e. you pay someone to use something). It can also mean “to let” (i.e. you lease something to someone) when you add an indirect object:

• Je loue un appartement. → “I’m renting an apartment.”
• Je loue mon appartement à un étudiant. → “I’m letting my apartment to a student.”

Louer is a regular -ER verb. Present-tense endings for -ER verbs are:
je –e, tu –es, il/elle –e, nous –ons, vous –ez, ils/elles –ent.
So for ils, you drop -er and add -ent, giving louent.

No. In most regular -ent endings (ils/elles), ent is silent. You pronounce ils louent roughly as [eel loo]. The s of ils is also silent before the consonant l.

In French, most adjectives follow the noun, but a small group of adjectives (often remembered by BAGS: Beauty, Age, Goodness, Size) generally precede it. petite is a Size adjective, so it goes before chambre.
Also, chambre is feminine, so you use the feminine form petite, not petit.

The preposition is près de (“near”). When de meets the masculine singular article le, they contract to du.

• près de + le métro → près du métro
  For feminine: près de la gare.
  For plural: près des écoles.

They all express proximity but with different nuances:

• près de = near, close to (no precise distance)
• à côté de = right next to, beside
• devant = in front of (facing)

Surround the verb with ne … pas, and change the indefinite article to de:
Ils ne louent pas de petite chambre près du métro.
Structure: Ils / ne + louent + pas / de petite chambre / près du métro.