I want to learn to play fingerstyle guitar like sungha jung. I found that most of his fingerstyle arrangements are made in different tunings and not the standard tuning. How does he choose which tuning to play in? Is any tuning easier than the other? Which is the most preferable for fingerstyle?
There is absolutely no "rule of thumb" for any specific tuning which would correspond best to any specific song. There is no tuning which is preferable for fingerstyle guitar. The most we can say is that a tuning may be useful to play a specific song, a specific arrangement of a song, or in a specific key. You may find it helpful to look to the following criteria for guidance:
Look at the key. If you find out what key the song is in, i.e., which 7-note scale is drawn upon to play the notes and chords of the song, then you can use that as a guide for what notes you want to have open strings tuned to. It is often the purpose of alternate tunings to make sure that the open strings are tuned to notes that are frequently used in the song.
Generally, the root and fifth of the key should be highly accessible. If the key of the song is "D Major", then the notes "D" and "A" will usually be easy to play using open strings. In most cases, the root and fifth note of a key can anchor the song to a particular key, so they can be used quite freely throughout the song. The standard tuning of a guitar is EADGbe, so if a song is in D, the lower E is often tuned down one whole-step to D, resulting in DADGbe. This is probably the most common alternate tuning, and is naturally suited to playing in the key of D.
Look at repeated bass lines or melodic motifs. If there is some repeated theme, bass-line, or other melodic fragment, the strings of the guitar can be retuned to make playing this motif more convenient, thus freeing you to do something more exciting and interesting with the remaining strings. In the example of DADGbe above, let's say the motif is a low "D" note followed by the "A" one fifth above. You can play this motif simply by playing the low 6th string and the low 5th string. That doesn't require the use of your left hand at all, and now your left hand is free to explore other ideas.
If this is not helping, then you probably should just start by learning some songs. It's definitely a good idea to experiment with different alternate tunings, but until you can play a few songs in different tunings, the use of alternate tunings may not seem clear to you. In fact, worrying too much about "what's the best tuning for me to use", or "how can I learn to use all of these tunings" is highly likely to distract you from the most important work which is just getting your feet wet and enjoying yourself. There's a website called Songsterr which has some of Sungha's arrangements.