how can i do something like this (personalized T-shirts)

At a guess it appears they will be using AJAX
I don't think Javascript would be enough on its own.

I don't want to scare you, because it is very possible, but goto their t-shirt design page (where you can edit the shirt text etc), right click on the page (not an image) and click "view page source" in the menu that pops up from right click.

The word *YIKES* springs to mind

It is a very involved large amount of work. If you are not already familiar with Javascript/Ajax you will struggle to 'quickly' learn all the required code to piece that sort of thing together.
Again, not wanting to put you off, you might learn fast. Another option is pay someone trustable to do it.
Or (**cringes**) ask that website if you can use their code - although good luck with that one, I would be very suprised if their answer was yes.

Hi, welcome to TWF!

You want to create a site like shirtcity.com, is that the question? Or do you want to recreate the custom t-shirt maker?

There are many people linking their websites to ebay listings and other auction sites. So I see it would be very possible, and can be worthwhile.

The facilities you provide on your site can range from very simple to complex.

If you don't provide customers with the facility to make their own designs like the site on the link you provided, then a simple website for your requirements would be very easy, and could be done with minimum HTML and CSS.
You could list all the garments you sell with an image, and then each colour and size for each garment quite easily.

If however you do provide customers the option to make custom designs then that can become a little more complicated, but not necceserily out of your scope.
It would be much easier to allow them to input text to go onto a t-shirt and then click "refresh", and that would place the text on the shirt where they wanted. It would be easier that way than the site you linked to, as it changes user input instantly without any page loading.

You've got three options:

1. learn how to make that application yourself, it's not easy and if you're a total beginner will take you at least two/three months of hard work;
2. pay someone to do the work for you, probably won't be cheap I'm afraid;
3. learn how to do the work yourself, but start out small and build up as your knowledge grows;

Option three is the best in my opinion. Just start out with a very basic form, that submits to a PHP script, that sends you an e-mail with the user's choices. Then expand it to allow the user to see samples of clothes in different colours/sizes. After that, look at dynamically creating images with the user's text on (you'll need PHP and the GD or ImageMagick libraries).

It all depends on what you actually need, what you know, and what you can afford.