When you visit your Webstore to look at an product such as a T-Shirt with variant sizes and colour you'll see a button with the product to 'Show Details'.
If you tap on that button you'll see an expanded view of the product where you'll have the option to select the size and colour variants of the product.
Once you've chosen your size and colour just tap on the blue button at the bottom of the screen to 'Add to Bag'.
In your shopping bag you can increase the quantity of the product you want, continue shopping or proceed to checkout.
Once you've gone to the 'Checkout' follow the onscreen prompts to make your payment.