Anyone here proficient in trading options? I'm still learning with practice trades with make believe money

. And I'm trying to familiarize myself with my online broker TD Ameritrades purchase tools...its a little different then the website I've been using. When I go to buy options it makes me choose between open and close. Is that a choice from the beginning of the market and the end? I can't buy them while the market is open? Also I have a little confusion with the write/sell choice... I just want to buy a simple call or put... I'm not buying yet just trying to learn more I'm trying to get my account activated for options trading now so I can utilize the Options 360 program! Any help would be appreciated!