Choice of instrument for writing songs
One of the simplest instruments to write brand new songs with is the electric guitar. You could also use a piano or keyword though but you can't take one of those out on a sunny day, sit beside a river and get inspired! Electric guitars can have a great sound but, again, taking them outside to write songs can be difficult. That's where an acoustic comes in handy. But if you're writing songs indoors then an electric guitar with an amplifier is a good choice. It's because of the fact that it's played by using an amplifier. Most amplifiers (amps) have many knobs which can be adjusted to get the kind of sound that you're looking for. Different sounds can inspire song writing.
If you choose a guitar to write songs then use an electric guitar with a good amp
With regards to guitar amps, you may commonly adjust all the level of bass, treble, gain as well as middle. Several amps have other settings that you can use to "dial in" a plethora of different tones, for instance reverb plus chorus. These can all help to inspire the song writing process. Reverberation, often shortened to reverb, is the form of very limited delay and could be used to include depth in the outputted audio. A chorus effect thickens the sound that the amp creates, it's almost like playing many guitars in sequence. A few amps available have built-in effects, which could add delay, a flanged effect and distortion. In terms of music, there are few things that are nicer than a driven valve amp which is cranked - sweet saturation, oh man! Although your amp may not include all of these effects, you could buy pedals which produce any of them.
Some guitarists who are singer/songwriters write the lyrics before that create the melody, whilst others might come up with guitar parts then whistle or hum over it before the lyrics even exist. Some use a mixture of both. Whichever method you choose just make sure that you enjoy the whole song writing process because writing songs should be fun!