Is there a site that can split characters into radicals? For example, let's examine the character 天 (heaven). It has a 人 radical (men). And then we add a strip to become 大 (big). And then we add another strip to get 天 (heaven).

Now, is there a site to do so?

I think that'll be the easier for me to understand.
The online chinese dictionary MDBG provides radical information for every character in its database. For instance, if you search for the character 天 (tiān) and click on the first result, the "Rad/Str" column reads 大 + 1, i.e., the radical 大 plus one stroke.

Zhongwen.com also gives information on character decomposition. The entry for 洋 reads "Water 水 with 羊 phonetic." 

As Alenanno pointed out, not all parts of characters are radicals, but these two resources can give you more information about how characters are constructed.