Its not exactly this simple, but:

cmyk -> for print

rgb -> for computers

If you are printing yourself, on something like an Epson, keep it RGB.  If you are going to a printer, first ask what they way, but assume it'll be CMYK.

Don't change any of your photos to CMYK until you are 100% done with them.

There's a thousand thorough articles and discussions on it.  Don't be scared, Google is your friend!

thanks @calculator & trace, for the pointers & encouragement.

which should i use for portfolio making? i will use outside printing service for the finished product.

currently, i've the setting to RGB, how do i go about switching it to CYMK for printing later?

will there be any adverse effects later on?

and @trace, is there a reason why i shouldn't change the setting of the working files to CYMK until I'm completely done?

appreciate it

Have you worked with CMYK?  As noted, there is no "right answer".  It takes experience.  CMYK will ruin some images and not change others.  CMYK is a smaller color space vs. RBG, which means some things can be seen in RGB that can't in CMYK (generally, CMYK destroys vivid blue skies).

You keep things as RGB because that offers the greatest flexibility later on.  

If you are printing via Kinko's, RGB will be fine.  If you are doing a full offset printing run (which I doubt you are), then you better learn some more advanced approaches (and talk to your printer).  You can convert in any program.  As I understand it, InDesign has a pretty good conversion engine (though I have not personally used it) for exporting to a print-ready PDF.

Again, Google is your friend.  Search, read, search, read, search read....

There are ways you can convert in PS that minimize the pain, like previewing your CMYK next to your RGB, etc.  

@trace, that was very helpful. many thanks

CMYK = offset printing (newspaper etc.)

Adobe RGB = regular printing (InkJet etc.)

sRGB = for screen.

If you're using CMYK or RGB for print, you may also want to calibrate your monitor. Which color space you use can be rendered moot if your monitor isn't displaying corrected colors to begin with. Even a software calibrator is better than nothing.

Also, all printers print using CMYK, even inkjets. A printer that claims to work in RGB is really using a separate conversion process at the printer level to approximate colors as closely as possible. The results may or may not be what you see on your screen. As trace noted, blues in particular don't translate as well as warmer colors.

If you want your CMYK images to "pop" in print you can increase the contrast through curve adjustment and use sharpening filters to simulate the effect of vivid colors (it will look a bit weird on screen). It'll be as close as you can get to the vividness of on-screen RGB images without using an expensive 6+ color printing process (CcMmYK isn't really a 6 color process, it only smooths out lighter shades to avoid half-toning).