Choosing a Song
Obviously, if you're going to create a chart, it needs to compliment a song. However, there are songs that can make good charts and there are also songs that are very difficult to make interesting charts for. This shouldn't stop anyone from attempting to chart whatever they like, but it's good to keep in mind, especially when starting out, so that you can work with songs that are easier to work with until you become more comfortable.
First of all, before you even think about whether the song is good for charting or not, you should like the song that you choose to make a chart for. If you don't like the song, the chart will be uninspired, and it won't compliment the positive aspects of the song as well as it could, and usually you will get tired and burned out partway through. Chances are, if you make a chart for a song you don't like, you won't make a good chart. Pick a song you like.
Apart from liking the song, the song should also have specific traits. The songs that can potentially produce great charts have certain similarities, as do the songs that are difficult to make charts for. Songs good for charting tend to be more dynamic and varied, not repetitive, to better keep the player's interest throughout the chart. Rhythmic variation is also important, as a song with very simple, straightforward rhythmic patterns don't leave many possibilities. Songs good for charting also have many other notable features that can add a lot of color to the chart, such as effects and embellishments, multiple melody lines and instruments, unique structure, etc. What separates the good songs from the really great ones are the details. A melody played over and over again is usually not as interesting as a melody with variations upon it each time it is heard again. Lots of different effects and fills give the chart artist multitudes of options when choosing what to follow.
Another thing to keep in mind is that what songs you choose to chart also depends on how you chart. Certain techniques are more appropriate for certain types of songs. Identify the techniques you are proficient with, and try to find songs that can really build upon your strengths. These are all general guidelines, not hard rules.
Try not to pick a song just because you found one particular aspect you wish you highlight. Often times, the rest of the song is ignored, resulting in a bland chart with few interesting sections. Try to stick to songs that you understand, that you truly enjoy listening to.