Okay, asides from being able to play guitar very well, songwriting is important to anyone that loves their sound and needs a foundation.
1) First, what kind of music do you play?If its jazz, you'd want to go for purposely whacky grammar. But if your a heavy metal kinda person, try things that describe either a defining life trauma or a gravedigger that robbed the wrong cemetery. Certain topics appeal to different styles. If you wanna sing hardcore death metal with jazz music be my guest. All I'm saying is pick your spots.
2) Then, it's about what you feel and when.Pour your heart onto the paper. Listen to what kind of music appeals to what your trying to write. You may conjure up some inspiration. All it takes is a little elbow grease (metaphorically speaking. Don't go to a store in the grease section for elbow grease applicants... jk)
Carry a notebook and pen with you everywhere. You never know when that groundbreaking idea will strike your mind. Even if your notebook and pen is a sharpie and your pants. (Hopefully not a good pair. Gotta do wat ya gotta do I guess.)
3) Writing and rewriting is a must...Masterpieces aren't written in five minutes. You would end up with a mess most likely. Take your time with it! Perfect its every little bit of machinery until your fingers bleed (or 'til you have something you are proud of.
Everyone knows it takes time to do things. If you want it great, work at it.
4) Add the music!Of course you don't want to sing the song to silence. Add your music as you go along. Keep everything in line. Even add your solo eventually. Every piece has its hook!
Hope you can take some info from this. Maybe inspiration! Just keep these four things in the back of your mind and you'll have all you need to write that top hit or private jam session song for the garage band. Thank you for reading! Keep shredding!