I started my baby girl on a sippy cup at 6 months and she loves it ,her Dr recommend the 360 cup which is awesome it doesn't spill if you turn it upside-down and easy to clean ,she drinks breast milk and water in it plus that cup will not mess up her teeth
I started my son using sippy cups at around 4 months . we started using the training nipple by tommie tippie then transitioned him to the spout . He's 8 months now and can drink from a straw so were now about to start him using the straw sippy cups .
As far as what to put in it . you can put water if you choose 1 oz . . I dont often give him juice but if I do its only about 1oz mixed with water (apple juice)
So far during the day I only give my son milk out of his cups except for naps and bedtime . He's doing great .
All of this depends on your child BTW because not all babies are the same as we know