Hi Sakura,
Glad to hear your branching out with your writing! Like others have said, it depends on the person, but since you're a beginner on guitar and piano here's what I'd recommend you do:
1. Start off by coming up with a VERY simple, 3-4 chord progression on whichever instrument you're more comfortable with. Use C, F, G, Am in any order that sounds good to you. Something like this:
C / / / | F / / / | Am / / / | G / / / |
or
F / / / | C / / / | G / / / | Am / / / |
(Each / means 1 beat, and the chord counts as a beat too. So | F / / / | is 4 beats of F)
You can also throw in Dm or Em for color if you want, but C, F, G, and Am (and their equivalents in other keys) are the most important chords in music, (and have been for hundreds of years). Start the progression on Am if you want it to sound dark or sad.
It doesn't have to be complicated to sound good. Most songs that become hits are painfully simple. You can just look up some chord progressions for songs that you like to get ideas too. You'll notice the same chord progressions, over and over, so just pick one you like best.
2. Play the progression until you get comfortable with it.
Find a tempo and stumming pattern you like and can work with well. If you have trouble with this just take a tempo/strumming pattern from a song you like--the song will sound completely different when you're done, so don't worry about plagiarism here.
3. Start trying to fit some of your lyrics to the progression.
You can do this by singing your lyrics while you play the chords, or by humming different ideas until you come up with something you like...then writing lyrics for it later. This is the hardest part for a lot of people, because it takes a lot of practice to get good at coming up with solid melodies, and then maybe words fit on those melodies complicates things even more.
The best advice I can give you for this part is to just keep practicing and have fun with it. Writing the melody and lyrics is where you really put yourself into the song, so for me it's the most personal, fulfilling part of the song writing process. It can also be the most frustrating.
4. Decide whether what you just wrote will work well as a verse or chorus.
If it's more low-key or scattered sounding it could work as a verse, whereas if it's high energy, catchy or powerful it will probably work better as a chorus. Just think of songs you know and how their verses and choruses sound.
5. Repeat steps 1-3 to write a complimenting part (verse or chorus).
So if you decided the first thing you wrote sounded more like a chorus, then try to write something that sounds more low-key and can be used as a verse. Again, just use your ear and think of songs you know and what they do. Keep the chord progressions and rhythm simple. Simple is always better.
6. Repeat steps 1-3 again, this time trying to create a part that will work well as a bridge (if you want your song to have a bridge, most songs do).
Your bridge should be a little different from the rest of your song...it gives listeners a chance to catch their breath before repeating the chorus again. AND it gives you a chance to be creative and write something different.
You can write a high energy bridge that tries to intensify the song, or a low energy one that tries to slow it down a bit. Whether it's high or low energy will depend on the chords you pick, the melody you write, and the rhythm you play.
That might sound complicated, but the key here is to experiment. Just write SOMETHING at first and see if it works when you play the whole song. If not, just write something different and try that!
7. Put all the pieces together.
I recommend this layout to start with:
Verse, Chorus, Verse, Chorus, Bridge, Chorus
It's time tested and has been working for forever. And, again, it's SIMPLE. If you want to change it up though, feel free to experiment! Just keep in mind that if you're trying to write a hit song you should use a layout that people are used to.
It's good to be different, but if you're too different most people will just think you're bad (Even if you're really good). People like hearing what they're used to. I know Jazz guitarists that could shred me to pieces, but no one buys their CDs because their music is weird and over people's heads.
And you're done!
Hope that was helpful, sorry I got a bit long-winded but I wanted to make sure you got a good view of the whole process. I know how confusing songwriting can be when you're just getting into it