Hi Emma,
Use a clean sterilised container and store it in the freezer. I used to use the avent bottles which double-up as containers.
When you express put the expressed milk in the fridge and then let it cool before adding it to top up the container in the freezer. You shouldn't add warm milk to cool or cold milk - you need to cool the fresh stuff first.
Hopefully, you won't need long to get enough for a feed. Just remember to relax when expressing - it helps with the let down.
Good luck,
GIDGET