To keep from giving off spoilers, I will not explain how River’s timeline with the doctor begins, but if no one knows her timeline and the Doctor’s run opposite of each other. The first episode that River appears in during the series is actually last time she will get to see him.
The first time that River meets the Doctor is during an episode where the Doctor is actually Matt Smith. To keep from spoilers, here is right after she starts looking how she does through most of the series.
Even though River is not around much during the series, she is someone during the show that adds a touch of mystery to the Doctor’s story. It does not happen very often that the Doctor meets someone who knows so much about him while he knows nothing about her. Most times during the show the producers are not good at developing the plot in the long term, but this aspect seems to be one that they thought out at least a couple episodes ahead of time. As soon as it is explained as to who she is and why she is around the Doctor and his companions, you will figure out her true nature. But..