
To be confident that you will definitely get the iPhone 5C of your own, if Apple.com crashes under the pre-order load, try the Apple Store app, and if neither work, start trying preferred carrier website. This is advices based on prior years’ pre-order experience.
However, the iPhone 5C pre-orders might not prove as popular as previous year's pre-orders as many still wait for iPhone 5S to come, so the chance of website crashing is lower. Unfortunately, it seems there will be no iPhone 5S available for pre-order. Quoting iMore, “Twice as many new iPhones to choose from means twice as hard to predict as previous years,” so who knows.
Wish you smooth pre-ordering process!