help me with shopping cart

Shopping cart depends on need I suppose.

If you have 2 weeks Ubercart is likely your better option since you know Drupal already and unsure of your PHP level. Of course, if you are doing Drupal 7, then you also have the option of Commerce (http://drupal.org/project/commerce) vs Ubercart. If you want to have the power and stature of Magento but not want the development stress, they also have Magento Go as a payable service option (http://go.magento.com/).

I haven't done anything in Magento but started learning and it has a high learning curve depending on your comfort level with PHP and understanding of MVC (Model-View-Controller) as well as OOP (Object Oriented Programming) architecture. This depends on what you want to do beyond what a straight community version install does (the pro is quite expensive for a small mom and pop store I would say). If you are familiar and know Zend Framework, which Magento is built on, than you already have a head start in learning. Magento however is configuration-based MVC versus the standard MVC so that is a learning curve in over itself. From what I hear, Magento can also be resource intensive so consideration would have to made for scaling depending on your shopping cart needs and traffic. If none of this sounds familiar, than definitely go the route of a Drupal shopping cart considering your timeframe.

Hope this helps some.

Between Magento and Ubercart, I'd go with Ubercart if possible. If you're already building the rest of the site on Drupal, it's the way to go. Magento is very powerful, but it's pretty slow without some juicy hosting (even more so than Drupal in my experience), and its theming engine is not as easy to build on as Drupal's.

You might also, given the short timeframe, and if "a shopping cart" is all your client needs (that phrase is very vague and can mask huge complexities, but we've all been there with clients), you might look at hosted solutions like Shopify or Magento Go. You end up not having to deal with PCI compliance in the hosting, and lots of other bothersome bits, at the cost of customization and so forth.

I've working on a Commerce in D7 site now, and done several sites in Magento. Here's my experience:
- Magento is complete, but can be harder to customize, much bigger, slower, with a higher learning curve. Theming is more limited.
- Drupal Commerce is less complete as an eCommerce solution but can be easier to customize. Theming is much more powerful.

If all you need is an eCommerce cart with no customization (besides theming), you're better of with Magento. Drupal Commerce has fewer eCommerce features and the backend is much more limited than Magento. You will have to do some work to get Commerce to meet your needs. Magento is the compete solution out of the box. Magento is designed to meet all standard eCommerce functionality out of the box and uses configuration to manage that functionality. Drupal Commerce meets only a minimal set of base functionality out of the box, and you must add modules and fields to add functionality.

As an example: the base Drupal products do not include all the fields you will need, whereas the base Magento cart has more than you need.

But if you are doing anything besides just eCommerce, you are probably better off with Drupal Commerce. Magento has very limited CMS functionality. It can basically create static pages. And it has nothing like Views, so creating a block that displays Featured products is custom code (although this case straightforward) or a module. Updating the admin order view is totally custom in Magento, and trivial in Drupal.

As a side note:
Because Magento operates entirely as an eCommerce cart, the code can be easier to work with than Drupal, once you know how. It's hard to describe this in a post, but working with the Magento Order, Customer, Address objects, etc. is easier than working with the corresponding Drupal code. The MVC pattern and Magento Objects works better here. Finally, there is much more sample code and modules for Magento than Drupal Commerce (I mean modules specific to Drupal Commerce, not Drupal as a whole).