hello @rupaibabu.. welcome to steemit 😁 you can upvote 10 times a day with 100% of your vote. when you have 500 sp a slider appears when you vote and youn adjust your vote from 0% - 100%. Then you can vote for example 40 times with 10 %. the best time to vote is after 30 minutes for better curation rewards
the value of the vote depends on the steempower you have.the same happens with curation rewards. You can vote an old post but after seven days noone gain any rewards neither the author or the voter.
resteeming a post can gain sp if someones votes for it. the users who resteem doesnr get any sp but helps the author of the post to gain more visibillity. hope I answered all of your questions. if you have more feel free to ask..happy steeming