Problems converting geographic to projected

Hi - I am a complete beginner using ArcGIS 10. I have a data set in WGS1984, but I wish to use the 'Calculate Geometry' function to find the length of a line, which I understand cannot be done with geographic coordinate data. I then tried to use the 'Project (Data Management)' tool to transform my data into VN_2000_UTM_Zone_48 (my data is from Vietnam, and I was told this is the correct zone for my area). However after I've entered that I wish the output to be in VN_2000_UTM_Zone_48, it wants me to use a drop down box to select which method. How do I know which to choose? I did find another forum post asking a similar question, which said to look up the help pdf file in your documentation folder listing the areas of each transformation, and I've done this - but it didn't help me!
Any suggestions on how to convert my data to projected coordinate so I can use the 'Calculate Geometry' function would be appreciated!
Thanks