Write lyrics and chords together when possible. Not all songwriters work the same way, and inspiration doesn't always cooperate. Sometimes, you'll get an idea for words without any kind of music. Sometimes, a chord progression will inspire you to start singing. If this happens, go with the flow and write. Still, one of the best ways to begin writing on an acoustic guitar is to pick it up and strum a few chords in whatever key you prefer. Keep pencil and paper nearby, and try singing a few lyrical ideas with the chords.
Write lyrics down as they come to you. Use a pencil so you can erase when you need to make changes. Songwriting is a process. You will likely change words and move chords around as you write your verses.
Place chord names above the lyrics. Regular notebook paper works well for this phase of the writing. You can write chord changes on one line and lyrics beneath the chords. When you place the chords above the lyrics, try placing them as close to where they belong as possible. Hyphenate your lyrics to help distinguish the syllable upon which a chord changes.
Add detailed notes in the margins of this beginning song draft. Unless the song is meant to be strictly acoustic, you will write other song parts later, maybe on another instrument or by allowing band members to compose their parts. Adding notes to your acoustic draft, such as the way you want the drum to sound, whether you want a piano solo or whether the chords are played by an electric guitar will help when you later refine your song.