Important Section. Reading this is highly recommend!
Bonsai require more water than the average house plant. They grow in special soil that is very coarse to encourage good drainage. Their pots are also very small which means the soil in them won't hold much water. Watering is a daily job when you own a bonsai tree. If you do not water your bonsai tree properly, it may die.
Ensure that the soil is always slightly damp but not drenched. How often you water your bonsai will depend on the weather, and how deep your bonsai pot is. Keep an eye on your bonsai during hot days. Bonsai in 6" pots or smaller will need to be watered every day, sometimes twice a day if the temperature gets above 25 degrees (you can combat this by reducing the amount of full sun to 5 hours a day for trees in small pots). A conifer bonsai will tolerate more sun before needing water when compared with a broadleaf bonsai. That isn't to say that your juniper (conifer) should be neglected.
Use your finger to check how damp the soil is before watering (in summer, the check can be skipped if you know it's going to be a hot day). If it feels a quite dry, then go ahead and water thoroughly. Wait a bit after watering, then water again. When watering, it is important to soak the roots completely. Wait until you see water running out the drainage holes at the bottom before stopping.
An example of how to water your bonsai tree correctly. Try not to keep the water stream in one place for too long otherwise the soil will wash away.