A conlang, or constructed language, has been a staple of the fantasy genre for ages. In a world outside our own, it often makes sense for the characters to use words that we don't have. This also is common in sci-fi, since aliens probably aren't going to show up speaking English. You might recognize Tolkien's elvish or Star Trek's Klingon as some popular conlangs at your local conventions. So writers have been making up their own languages for many years.
For you own book, you probably don't need to create your own complete and functional language. For most stories, a few made-up words sprinkled in from time to time will add a lot of flavor and realism.
One of the easiest ways to start making up words for your fictional cultures is to define the language's sound system. By choosing a set of sounds that are used by the language, you can make a language sound distinctive as well as sounding cohesive. While most readers won't know much about linguistics, they can still hear the difference subconsciously. If you hear the word "aloha" or "bonjour," its easy to tell which one is Hawaiian and which one is French. They have different sounds and feels. And this is partially created by their rules of sound.
Assuming you are writing in English, I highly recommend you stick to sounds that can clearly be written in the existing English alphabet. This will make your life much easier and make your language feel less like gibberish to your readers.
You can choose the sounds for your language in lots of ways. Maybe you will want to write a list of "words" that have the sound you want and reverse engineer the list. Maybe you could look up the available sounds (often called phonemes) in a real life language that you like. Perhaps you could look up the entire International Phonetic Alphabet and choose sounds that you can realistically spell for English speakers.
However you go about choosing your sounds, make a complete list of them for your records. Don't forget to write out each sound and not each letter. For example, don't use c for both s and k sounds (although can choose to use c for either). Add a separate entry for sounds like ch or sh.
List each possible vowel sound separately. English may only have a handful of vowels in the alphabet, but we have a ridiculous number of pronunciations of those letters. Try to write out the vowel sounds in a way that makes sense to you, such as "the a sound from cat" or "ah," "oo," etc. This way you will be able to keep your own sound system straight.
You may want to play around with your spelling a little bit too. English is notoriously bad at spelling things phonetically. So choose how you want to represent your list of sounds in a way that makes sense to you and adds flavor to your conlang. Maybe "kw" is your preferred spelling instead of "q." Maybe you use "oo" to make a certain sound or "ow." Just make sure not to get too far away from pronunciations that exist in English so that English speakers can still comfortably read your conlang vocabulary.
Once you have all the sounds you want written out, you'll want to sort out the vowels and consonants. This will help you turn your list of sounds into actual usable words.
You can now generate words by combining these letters. First you'll want to build syllables. You can do this by pairing your sounds: one consonant to a vowel, a vowel by itself, or consonant vowel consonant. Using syllables with this structure you can build words of 1-3 syllables. I recommend making a long list of combinations at random and then choosing your favorites to assigning meaning to and use in your book.
If these words are enough for your purposes, then congratulations. You're good to go. If you want to get a little fancier (something that might be helpful of you're going to eventually have multiple conlangs), then you'll want to spend more time creating rules about how your sounds can be combined. In my second post on making up words, I'll walk you through some steps to help you choose which sounds are allowed to go together and in what ways.