Hello,
I'm a first-time mom, too, and our baby just turned 1. She stands in her crib all the time...it's just something new she can do and so she does it. I know she's safe, so it doesn't bother me...and eventually she does plop herself back down and babble herself to sleep. If she's really upset, I wait a few minutes (between 5 and 10) and go back in to put her down, then walk out again...usually by the time I'm at the door, she is already standing again. Don't give it too much thought...it's just a new trick and it will pass but I know how frustrating it can be, and it seems so sad to see them just standing there, looking so helpless!