Get your script color your characters lines one color and the lines before your lines another color.
You will need to memorize the lines before your lines too so you can be sure you are saying the right line at the right time. It's a bit of extra work but it really pays off!
Read thru your lines every day try to read them before you go to bed.
Grab a friend (preferably someone that is also in the cast) have them read your cue lines (the lines before your lines) to you and reply with your lines.
Now when you start running lines at first you will probably want to just mark thru lines (with no character) once you have it mostly memorized you can start running thru it like performance
- A script