What can I do after learning to spark

In The Two Towers, why was √Čomer so hostile to Aragorn, Legolas, and especially Gimli when they first met?

There are two basic answers, both different but true.

1) As Eomer makes clear in the book and in the film, times are strange and it is difficult to know who to trust, especially in the face of Saruman's betrayal. Here are three people entering the country. It was not a situation (in Eomer's judgment) that required courtesy. Instead, he wanted answers.

2) A more overarching answer is that a major theme of LOTR is that confronting and fighting real evil requires solidarity. No one, no race, or country can do this alone. This can be seen when all races / persons present in the council report something about the ring in Rivendell. The amazing thing here is that a person from the "lowest" of all races, the hobbits, is willing to risk it all and wear the ring to Mordor. It is Frodo's sacrifice that gives rise to a "community" made up of all races: hobbits, elves, humans, dwarves, and even a wizard (which Tolkien readers know is truly a heavenly representative).

So there are instances of "racism" or "abuse" (to listen to the OP) to remind readers that races are fragmented in the narrative world of LOTR, and this lack of unity is a major threat to their chances of overcoming evil.

Although this concept is ubiquitous in LOTR, I think it is a good example when Frodo escapes Boromir and lands in the seat of sight. When this happens he can see miles and miles around Middle-earth, and he sees every region fighting the armies of evil in what basically seems like a lost cause. Middle-earth is divided and thus falls.