I need some help, i need to do a Pop/Rock song, like Paramore or All time low, something like that, on guitar, but i don't how to start.

Do i follow a rythim first using a chord progression, or where i put the mini riffs, how to do riffs that fit in the music and music in the riffs....

Start with whatever comes to you first. You can start a song using a bit of a riff, some lyrics or a vocal melody as long as it's strong enough to work from. Listen to the artists you're trying to emulate first and then try and pick out things that define their songs. Then just go for it and see what comes out
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the first thing i do when i write a song is i listen to music i want the song to be influenced by... then i sit down and think and write a poem. you can even write as you go along during your day. if you hear a phrase that you think is awesome or would sound perfect with what you are trying to say, write it down. then i put music to it. you can even do it the reverse way. find individual notes and then chords and build from there. writing a song is writing a story with only a few lines. you have to find the best way to convey what you're saying. hope this helped
I've just wrote a song for the band which is currenly nicknamed "pop-rock". You have to stick to the straight major/minor keys and write a riff that is simply but catchy.

Look at the tabs for all time low & paramore and look at the guitar playing style.

I've usually got a rule that if you cannot play it on acoustic then it isn't pop rock
But I how i fit a riff or a arpeggio, to sound good with the rest of the song?

thanks for your help, and sorry my english
Quote by meco66
But I how i fit a riff or a arpeggio, to sound good with the rest of the song?

Start off with finding a good key for the song (some common pop/rock keys would be C, G, and A). Any riff you write in that key will sound good w/ the rest of the song. Same thing w/ arpeggios.

Other that that, do arpeggios in the slower sections, and try to capture the "feel" of the music in the riffs. For example, if your song has a climax, write a riff that starts low and soars high. Or for the end of the song, start high, go low, and end with a power chord.

Got a little long-winded there, but I hope this helps a bit.