You don't. The best you can do is trade for it or start with bulbasaur and level it up to about level 32-38
You'll need to evolve Ivysaur to obtain Venusaur.
Yes your rival will if you choose squirtle, i have a venusaur.
You have to trade it from another game.
Trade 1 from another player or migrate a bulbasaur, ivysaur or the venusaur from leafgreen or firered
no. unless you hacked the game, that would be cool.
Venusaur, Exeggutor, Meganium, Sceptile.
Your rival will get a venusaur only if you chose squirtle as your starter i believe he will get one when he is at silph co, other than that to see or get a venusaur a friend with firered or leafgreen needs to give you one.
You must migrate Venasaur from FireRed/LeafGreen to Diamond.
you cannot catch venusaur you need to trade it from someone
You can't catch Venusaur on Firered, but you can get Venusaur by choosing Bulbasaur as a starter then training it. You can trade it (from someone who chose it as their starter) or you can use AR or Gameshark to catch one. If you know someone who chose one as their starter but don't wanna give it away you can ask them to breed it with a ditto and give (trade) you the egg. It will hatch into a bulbasaur =)
Look at this page: http://www.serebii.net/pokedex-rs/003.shtml
If you chose Charizard then there is no way to get venusaur unless you have an AR or you trade from another game. I suggest trading from another game if you can just because it will grow quicker.