Learn Russian: Это участок, на котором мы будем строить гараж. - This is a plot on which we will build a garage.

The relative pronoun котором refers back to the masculine noun участок (plot). Because we build on a plot of land (на участке), we use the preposition на. Since на here indicates a location, который takes the Prepositional case: на котором ("on which").

Why is the verb будем строить in two words instead of using a perfective future verb like построим?

Будем строить is the imperfective future tense. It translates closer to "we will be building" or "we are going to build". Using the imperfective here emphasizes the ongoing process or the general plan of building, rather than just the final, completed result (which would be the perfective построим).

Is гараж in the Nominative case here? It doesn't look like it changed.

It's actually in the Accusative case, because it is the direct object being built. However, because гараж is a masculine inanimate noun, its Accusative form is identical to its Nominative (dictionary) form, so the ending doesn't change.

What kind of "plot" does участок refer to?

Участок is a very common word in Russian for a piece of land or property. It typically refers to a lot meant for building a house, a dacha (country house), or for gardening.