A quick way to learn lines is to record all the lines (and your cue lines) and play them back and say them along with the recording. You can record with your computer, phone, etc. It's important to hear the cue line (line before yours) so you know when to say your line. Play it over and over and say your lines along with the recording. Then when you know it better, pause the recording after the cue line and try to say your line. Listen to the line and see if you got it right and try again.
Other ways that people learn lines include reading the script and covering your lines up. Read the cue line and then try to say your line while it is covered up. Another common way is to have someone read the cue lines and you try to say them. I have even seen people type all their lines as a way to memorize. Look at the cue line and then try typing your line without looking at the script. Everyone memorizes differently so it is good to try different techniques to see what works best.
And right before you perform, you can do a speed through with the other actors. Sit or stand together and go through all the lines as quickly as possible (don't worry about movement). Have someone mark down the lines you miss and read them over and over before you go on.
Find more acting tips at: http://www.freedrama.net/acting.html