Q: Loopy Start

what is the best way to start or reboot my MacBook Pro in recovery mode? After installing OSX yesterday my computer has been looping at startup even in safe mode. I'm finding it difficult to jump in in the amount of time it allows me, before it shuts itself off and starts to reboot system all over again. I have already tried the shift key and that doesn't seem to work at all.my warranty, well my extended warranty that I bought, expired 2 months ago. Parentheses go figure! I truly hope that this does not end up as a very expensive fix. Murphy's Law being what it is, I just want to be able to fix my computer.

Is it possible to run diagnostics in MacBook Pro as a "guest"? I have run into a wall, it seems, and my profile starts runs halfway through, and restarts on its own. I will not allow me to override with "Command and R codes."What exactly do I have to work wth in SafeMode?