Its probably easier if you knew the piano part by heart, I believe it takes more concentration to sing. I’ve never seen someone looking at the sheets/notes while singing. Practice the song with your fingers, then move onto integrating it with singing.
References :

It just takes practice…for me I had to know the piano part really well before I could sing along with it. If I had to keep stopping to remember what to do next, I couldn’t really sing like that. Just learn the song completely, then try to sing with it.
References :

I almost never memorize the piano part if I’m going to play and sing them both at the same time. I sight read both. However, I’ve had tons of practice sight-reading. But I can remember playing and singing Air Supply songs in 6th grade. Just keep working at. It’ll get more and more natural. For me, it’s actually easier to memorize the vocal part, if it helps.
References :

Practice practice practice. I find it helps if I have already heard the song sung and played so as to hear how it would sound. Memorizing the vocal part I find easier — make it an automatic subconcious thing. So you don’t even have to think about it directly. Piano is what I focus on more.
References :

Start by playing something simple (that you know well & have memorized the lyrics) and singing the lyrics at the sametime. Before you start analyze the music and make connections between the piano and vocal rhythms and melodies. Take it slow. Then try it again a little faster. Continue until you are up to tempo. Then try another piece. Pretty soon you will get comfortable with it.

Another helpful exercise is to play a song while saying the alphabet. It’s like practicing hand-eye coordination for sports, only it’s hand-mouth coordination.

Good Luck!
References :

Its probably easier if you knew the piano part by heart, I believe it takes more concentration to sing. I’ve never seen someone looking at the sheets/notes while singing. Practice the song with your fingers, then move onto integrating it with singing.
References :

It just takes practice…for me I had to know the piano part really well before I could sing along with it. If I had to keep stopping to remember what to do next, I couldn’t really sing like that. Just learn the song completely, then try to sing with it.
References :

I almost never memorize the piano part if I’m going to play and sing them both at the same time. I sight read both. However, I’ve had tons of practice sight-reading. But I can remember playing and singing Air Supply songs in 6th grade. Just keep working at. It’ll get more and more natural. For me, it’s actually easier to memorize the vocal part, if it helps.
References :

Practice practice practice. I find it helps if I have already heard the song sung and played so as to hear how it would sound. Memorizing the vocal part I find easier — make it an automatic subconcious thing. So you don’t even have to think about it directly. Piano is what I focus on more.
References :

Start by playing something simple (that you know well & have memorized the lyrics) and singing the lyrics at the sametime. Before you start analyze the music and make connections between the piano and vocal rhythms and melodies. Take it slow. Then try it again a little faster. Continue until you are up to tempo. Then try another piece. Pretty soon you will get comfortable with it.

Another helpful exercise is to play a song while saying the alphabet. It’s like practicing hand-eye coordination for sports, only it’s hand-mouth coordination.

Good Luck!
References :