After spending 6 to 7 hours at school which is a long time for your first grader to be away from home, you are curious to know how her day at school was. On asking, she may just say it was OK. You may get to hear the same frustrating one-word answer when you want to hear more from her. For your child, to sum up an entire day will be hard so she may settle down for a brief answer.
What you can do
To get your child to talk about school, give her time to settle down, once he/she is back. Give her lunch and half an hour of rest. Meanwhile you can go through her notebooks, and check for any notes in the almanac. Based on this, plan the homework schedule. After you have eased the transition from school to home, your child may feel more inclined to talk about school.
Hear her out first. If she is reluctant to share, ask specific questions like: what was fun at school; did you talk to your peers; what subjects were taught today; any concerns that you would like to share etc. Praise your kid for any good effort. Ensure you give him more your full attention and put aside anything that you were doing at that time. Use informal moments like while walking in the park, car trips etc to talk to your child about her day at school.