I want to know if I can migrate my pokemon from sapphire to diamond. and if I can then how. and I migrated then may I will be able to choose a starter pokemon.please help..
Yes you can migrate your pokemon. This question explains how: http://pokemondb.net/pokebase/877/would-migrate-pokemon-emerald-version-into-game-like-diamond
Not sure exactly what you mean about starters, but if you restart your Sapphire game then yes you will be able to migrate the other starters across.
You will need to get to the point where you can buy PokeBalls and catch pokemon because you must migrate exactly 6 pokemon each time. That takes at least 30 minutes if I remember correctly. You can just catch 6 Zigzagoons or whatever, then migrate 5 of them and your starter.
Why yes you can, but the thing is that you need to get rid of some HM that your Pokemon learned in Sapphire and then get the National Pokedex on your Diamond then "Ta da" you can Migrate your Pokemon from the 3rd Gen. to the 4th Gen.