I want to start learning piano. My friend said all my guitar lessons sheets can be played on a piano and that I would not have to buy sheet music for the piano. Is this true?
It depends on the music sheet. Piano music often has guitar chords listed above the grand staff, so you could have been using piano music for guitar. If you’re using made-for-guitar lessons or lead sheets then its probably not appropriate for piano, though you may have some melody lines you could play there.
As a very rough approximation, if you have a grand staff (both treble and bass clefs) it may be designed for piano. If you have only treble clef then it’s not designed for beginner piano: it can be played on piano, but that requires more advanced improvisational technique. If you have tab or just chords and lyrics then it’s really not designed for piano. Again: advanced players can improvise it, but it’s not a good place to start for beginners.
Whatever you do have, if you’re just starting piano you’ll probably want music designed for beginner pianists. Getting both hands coordinated will take some practice and you’ll want to ease into that!
In addition to the other answers, I would say that music written expressly for guitar is usually going to be different from music written expressly for piano, even in the same piece of music, because the instruments are different. Guitar and piano have different strengths and weaknesses, and this is reflected in the music written for them, if it's at all idiomatic (fitting the particular instruments). Therefore, while you might be able to play and get some of the same effect from music written for another instrument, you will use the characteristics of your instrument best with music written (or arranged) expressly for it.
Piano music is written using both G (or treble) clef and F (or Bass) clef. Because of this you can absolutely play your guitar music on the piano. Guitar is written in treble clef but in a different octave than the instrument is played in.
The problem you will run into is that you will not have anything for the left hand to do, or you will have to try and split the task of playing complex songs between your two hands. This is a added burden. If you are a beginner you're better off learning to read both clefs. The same could be said of violin music or bass music, they can all be played on the piano but you have some missing information to figure out. If the songs are mostly single lines then you won't have a problem. If you have single lines with box chord charts above then as long as you know how to play the chord on the piano you can get by. But classical guitar music will have as many as 6 notes at once and possibly spanning a few octaves. Here you will need to figure out what to play with the left hand whereas in a piano arrangement this would be in the bass clef.
Two separate issues here. LEARNING guitar and piano and PLAYING 'songs'.
Guitar and piano have different techniques, different hand positions, different approaches to playing scales, chords, melodies... You need to learn guitar from a guitar tutorial, piano from a piano tutorial.
Once you have some fluency on each instrument and want to play 'songs' the same music copy will very likely do for both. Popular music is normally presented in 'Piano/guitar/vocal' format. You get the melody (and words). You get a rudimentary piano arrangement. You get chord symbols so a guitarist can strum along.
But there's 'songs' and there's music specifically for each instrument. If you want to play classical guitar, or even to play the exact 'licks' used on a pop song, music written specifically for guitar will be needed. Tab notation might be used.
And if you want to play Beethoven's piano sonatas, or Fats Waller 'stride' or the exact piano part from a Coldplay song, you'll need music designed for two hands on a keyboard, not one hand on a fretboard.
Beyond the answer of Rob (upvoted), I want to add that especially for beginners for piano, for melodic parts (so separate notes, not chords), have a number denoting the finger number (1-5) to show which finger should be used. This is especially useful when more than 5 notes are played which go up and/or down more than 5 (semi) notes. So one knows where to place one finger up or down another.
Of course this technique is not used (or at least different) for guitarists and not denoted on (beginner) guitar music sheets (neither on normal or advanced piano sheets btw).
Guitar music is essentially written on treble clef only, actually an octave higher than it sounds when played on guitar.
Of course this music could be used when piano playing - just bear in mind it will sound an octave out from the guitar. This can easily be remedied by playing the piano an octave lower.
Piano music essentially uses two staves, called the grand staff, so there's something to play with the left hand. That won't be present on guitar music as a rule, so you'll end up playing something like a melody line, if single notes, or a more complex right hand piano with up to six notes simultaneously. Quite a feat with r.h. piano!
So, yes it'll work but not be very satisactory except for the very beginning of your piano playing career. There's plenty of piano music available to download, and that, or other stuff you obtain, will be far more apposite for you as a piano player.
An example of your guitar 'sheets' would help, but I think some general comments can be made.
Guitar clef is normally treble or "G" clef. A pianist can read that clef. Well... a good pianist should be able to read various clefs.
Your sheets may use the G clef and be readable by a pianist.
Your sheets may be guitar tab or lead sheets both of which can provide chord symbols. A pianist could ad lib from those sheets.
If the melody is included - like on most lead sheets - then the pianist can play the melody with an ad lib accompaniment.
If the sheet is only tab with chord symbols, that's a hard case. I think most pianists will not be able to read tab and the best they could do is ad lib from chord symbols. They could take some queues from the tab in terms of rhythm and general figuration (scalar versus broken chord figures.)
I think these possibilities cover the typical guitar sheet music I have seen and you might get some use out of your guitar music on the piano.
Having said this, you really should get specific piano music. You can get either some method books or albums of beginner level material. The point of such material is to give you standard fingerings and to train the hands to play separate parts. You won't get those training specifics from guitar music.
Pick things according to the style you are interested it. If classical, there are lots of methods like Clementi's Introduction to the Art of Playing the Pianoforte. For pop styles look for books like those from Mark Harrison. Of course you don't have to use those particular books. My point is don't just buy any old sheet music. Look for stuff that matches your goals and suites your taste and learning style.