--> Transistor datasheet help

[Zero the hero]

Here's a question that I would have answered before...
Here you find the data sheet of 2sc1815 transistor http://www.microelectr.com.hk/datasheet-list/transistor/2SA%202SB%202SC%202SD%202SK/2SC1815.tif it's a tiff image.
There are many voices in the table, such as Vcbo, Ic, fT, Hfe, Ptot, Vces and many others.
If I need a replacement (an equivalent) of this transistor, which are the most important data (amongst these written in the datasheet) should I check to see if 2sc1815 and the other transistor are correctly equivalent? Can small variations in these data somehow be correct modifing the application circuit?
More shortly: how to use datasheet correctly?
Take for example the BC107B datasheet
http://www.microelectr.com.hk/datasheet-list/transistor/BC%20BCY/BC107B.tif Is it a good replacement? Why?
Thank to all.
Zero.

[Mark Hammer]

I use a much less rigourous and scientific approach.  I look at other pedals that use the transistor I'm trying to replace, and see where they use it.  That will often give an idea of what a suitable replacement is.  In general, I find a BC549 or BC550 or 2N4124 or 2N5088/89 easily replace a 2SC1815.  Just keep in mind the different in pinouts.