Determine long-term capital gains on Robinhood

Hi,

 

I'm currently investing on stocks through Robinhood. I'm thinking to invest long-term to reduce taxes. I'm trying to keep an asset for a little more than a year and sell it.  Because I'm buying stocks from a company multiple times throughout the year, so some shares are new, and some are older. How do I determine whether an asset,  or a number of shares are long-term or not?