Where to start when writing Guitar ???

None of the above.

If you want to WRITE music, you need to let it come from a pure place in you, not angus young, billy whatever his name is, or Dave Grohl.

Just fiddle with the guitar on your own. When you find notes or a position that you find interesting , write it down and play with that riff you made up.

you start with a good idea you can hum... you start by not trying... when you become inspired you will hum those notes and then find them on aninstrument because you will be compelled by the originality of the idea

if you're fishing for ideas then stop.... the hard work end is the screwdriver work, the initial theme(s) should flow naturally out of your head, fully inspired and original

to be fair I have forced myself to write some stuff for contractual reasons, but its never as good as the music I write by waiting for inspiration and then pouring all my effort and inventiveness onto that little kernel of idea until I have something I can feel proud of

also, stop thinking about writing on guitar, its just writing.... a good musical idea can be framed more than one way.... if you have a guitar centric idea that will only work in the style of 'angus' or whoever then tis probably a weak idea (though once in a while one gets lucky with something specific having universal appeal, I never have though)

the more you work on finishing songs, the more inspiration you will find you have and the more easily you will be able to develop them.... the bands you like come from a hodge podge of pop songwriting traditions so you may wish to research song construction and the history of songwriting from troubadours to hank Williams to the beatles to Motown to emo

You just play around with your instrument. Hit random notes, make patterns, think of a tune in your head and try to replicate it on the guitar. You will eventually hit a pattern that sounds cool to you.

I disagree ALMOST entirely (think of a tune in your head is 100% the way to go), the random style is one way to write a riff, but focus and intention is 9 times out of 10 better.... the random approach is cool for phrases in solos though

not everyone is a writer.... anyone can develop writing technique, not everyone has ideas

when writing parts (arranging) attempt to hear the finished song in your 'inner ear' as you imagine it and then try to isolate a part, say the guitar, and focus on it until you can hum or sing it out loud... then approach the instrument and make it come out of the instrument... you might improve it while playing with a hint of random luck changing a note or phrasing here or there through happy accident, but the core of that idea should come from inside you, not outside

small accidents and bits of randomness may be a disguised intention, a musical Freudian slip, but just fiddling around aimlessly seldom yields great music for most musicians though I admit there are exceptions

Boom is telling you to start with your hands... Jim is telling you to get your lips on it. Dirty lads.... The simple truth is the stuff you write will tell YOU the instrument it belongs on.

When you pick up the guitar and say, "ok this will be the guitar bit", put the guitar down and walk away. Not natural.

Do you have a smartphone? Grab a free microphone app... I use MicPro on apple http://www.downloadcollection.com/graphics/largeimages/mic_pro___recording-501510.jpeg I use this ALL THE TIME. If I am out and about or just playing with a few minutes downtime, when I stumble across some magic I slap the app and record a chunk for when I have more time. Sometimes I just acapella that riff into it.. hum or scat or bad beat box.

I was taking the fam on a bike ride one day and I got a solo in my head for a piece I had been working on, recorded it by mouth and got home an hour later and the pitch I had hummed at was perfect and it became the solo for that song.

Writing is personal, even riffs... how do you write an original song? You find some combo of notes that sounds good to you, that doesn't sound like anything else!

Sometimes noodling around will give you a riff, sometimes using a particular effect or effect combo will trigger something. Sometimes a capo or a drop tuning, or a weird tuning that you just happened across. Try a new chord, a new timing, or just follow a thought pattern. There are endless ways to write songs...

Inspiration by a particular song, and emotion, an event, a sound you created may become a focus. You may replicate the sound a vacuum cleaner makes, or a grinding machine, a bird whistle, a video game sound.

Writing isn't about writing. It's about being able to hear what isn't there yet, decoding clues that aren't given and bringing something to life that never even knew it was going to exist.

Magic is a cliché word.

Don't write. Just be aware of what you have done and wait for what you haven't thought about doing yet.

It isn't about making sense. It is about making music.

agreed! I used to use tape Dictaphones in the 90s and then transcribe to my staff paper notebook alter to think about it in an even more abstract way... being odler and more skilled now I tend to write it directly to paper (without touching an instrument if possible) -- sometimes I will also just have an idea, start humming, grab a guitar or sit at the piano and start playing along and the song pours out because it was up there all along.... other times its notes and months of consideration in the back of my head before I touch an instrument like I mentioned earlier

I think writing is a lot of waiting in the beginning and then a lot of individualized screwdriver work to finish something to a point where you can even think about how to present it as a finished live or recorded piece

I find it VERY difficult to describe my process to people, its veryitnernal, slow but effective.... its easier for me to tell people how it fails to work for me and how I've seen a lot of other aspiring songwriters I know fall on their faces. In order to help you figure out what will work for you I find it best to tell you what usually doesn't work. Although it entirely depends what standards you are aiming for. In general when I've had 'noodlers' present me their song attempts in band situations they are telling me its not very good before I can even voice my negative opinion and I can just answer with a nod and ask them if they want constructive criticism or advice to start over from scratch.

but recorder app, YES

the whole ehading here, writing for guitar? wrong ehaded! writing is writing.... music exists independent of instruments and is an almost mathematical construct.... I have a real Platonic view of music in that I feel it exists like Plato's ideal forms (like a perfect right triangleor the idea of a just government). If you are writing FOR GUITAR you are not in the world of forms and you are trapped in Plato's cave trying to compose with the shadows of the physical world. True art will be created with pure ideas that can then bend physical reality into the creator's intended form and will likely have more appeal to the author and the audience. But I am a nut, YMMV

Angus, if you are unfamiliar with Plato, look him up.... writing music is a highly creative and exciting entrée to the abstract world of ideas and I envy anyone just embarking on this intellectual and creative journey

writing is an act of sheer intellect and will that affirms your humanity and makes you feel alive... also, getting ones songwriting pat has other, non-intellectual benefits