How much
3 to 5 g per day. Loading with 20 g a day for a week saturates muscles faster, but it isn't required — a steady 5 g gets you to the same place in about a month.
Timing doesn't matter much
What matters is consistency. Take it with whatever meal you never skip. Rest days included.
Water retention
Creatine pulls water into muscle cells, which is part of why it works. An early kilo or two on the scale is intracellular water, not fat.
The kidney worry
It comes from creatinine being used as a kidney marker — creatine supplementation raises creatinine without harming kidney function. In healthy people, the evidence for harm isn't there.
Buy monohydrate. It's the form that has been studied, and it costs the least.