If a person is very sick, praying may be the only thing you can do.
But maybe it is hard to know what to say.
Start be lighting a candle for them.
Here are some things you could say:
God, please be with NAME.
Heal them. Help them to get well. Help the doctors and nurses to fix WHAT IS WRONG
Look after their pain.
Help them to know that we love them, and want them to get well and strong again, and to come home with us.
Or if it is their time, please help them get to heaven, and help us who are left behind with the things we need to do.
Some people say the Our Father or the Hail Mary lots of times.
Some people say a short prayer over and over eg "Heal our friend a make him whole" - this is called praying with a mantra.
If they are dying, light a candle once a hour.