What if a fielder stand on 30 yard circle? Is he called an infielder or outfielder. I have also seen that some player also stand some steps outside of 30 yard circle. So if do not walks inside thd circle while the bowler delivers the bowl, is he counted as a fielder outside the circle?
For one-day internationals, being on the 30 yard markers makes the fielder an infielder. To quote from the 2014-2015 playing conditions, Law 41.2.3(a):
During the first block of Powerplay Overs [...], only two fieldsmen shall be permitted outside this fielding restriction area at the instant of delivery
During the second block of Powerplay Overs only three fieldsmen shall be permitted outside this fielding restriction area at the instant of delivery
Note that this also answers your second question: the restriction is enforced an the instant of delivery, so fielders "walking in with the bowler" is absolutely fine so long as they are on or inside the 30 yard markers when the ball is delivered.