About 6 months. Most babies can bear weight when held upright at 6 months of age. I would not expect your baby to be able to pull himself into a standing position at that age however. Also, it is likely not until 9 months that most babies are pulling to stand and then cruising around on their own.
Usually 4-6 months. It is fine to hold your baby in a standing position as soon as she has good head control, usually around 4 months. There is a myth that standing a baby is bad for the legs or back. The more they stand, the stronger they get.
Depends ... Every child develops at a different rate - walking, talking, etc. Walking depends on development of muscle strength and coordination and balance. Heavier babies can take a bit longer to develop enough strength to hold themselves up. Einstein didn't begin to walk until 18 months, whereas some babies begin as early as 9 or 10 months.