If you were a male peasant (taking this to mean poor farmer), joining the military could be an option. As u/XenophonTheAthenian has described in this thread soldiers were typically or a rural background, and ordinary men could rise to the rank of centurion though how often this happened is discussed here by u/iguana_on_a_stick and u/Alkibiades415 (the latter has also described how centurion careers and retirement might look like here). The more typical social climbers in Roman society were freedmen, however