Need help with learning linux and how to put it on my mac.

[bigMAC28]

I want to be able to boot linux from my mac. I would like to use the OPT key on startup to decide which to boot from. I am having speed issues with my mac so i plan on doing a clean install this weekend. the way i would like my 500gig harddrive setup would be

75% OSX ML
25% Ubuntu ( or other version of linux depending on the best one, Suggestions)

I am very new to linux, and would like to learn more about it to further my career. Hence, why I am installing linux. I know there is a way that you download a special app that runs on startup that you have the option on which OS you want to boot from. I don't want to do that. I tried googling this issue and it seems WAY too confusing. Any help would be appreciated.

-Mike