Please provide more detail -
1. What version of Oracle and OS you are running?
2. In what setup you are running your database (parellel server, stand by, replication etc.)
3. What infastructure you have? I mean, how much memory, size of SGA wanting to start, swap etc.
4. have you ever managed to start the same database successfully? or it is first time after installation?
5. If it is first time startup, did you install it correcty as guided by Oracle docos? did you set kernal correctly?