problem transistor selecting

A simple answer is by them characteristics but unfortunately this is one of the most serious thing that an electronic engineer has to deal with.

Rather than trying to go through one or two semester's schooling here, perhaps we could point you to some resources if you can give us some idea of what you want to do, and perhaps also describe your level of knowledge of semiconductors so we don't insult you or waste your time with stuff you already know.

If you just want to switch a small load like an LED, the specification process is quite simple, but if you are trying to build a low-noise microwave communications amplifier, it can be rather complex. My usual procedure is to define the characteristics of the transistor I'd like to have for a particular part of the circuit, then check several manufacturers to see which of their products comes closest. If necessary, I'll make design trade-offs to accommodate what is available. Most of the manufacturers I deal with have parametric search databases on their websites to make it easier. After that it comes down to price, guarantee, and availability.

Good answer. The OP may not feel that you have helped him, since you have not revealed the secret component selection guide for non engineers, but that's pretty much the process.

The real question is, when to call the design good enough that you can quit looking for that one component that would make it THE perfect circuit.