、。
When I speak in front of a lot of people, I get stage fright.
Literal
I [topic-は] many [possessive-の] people [possessive-の] front [at-で] talk [object-を] do-[when-と], get-nervous-[completion-てしまう]-[explanation-んだ].
大勢 ('many people, a crowd'). 人の前で ('in front of people'), で for location of action. ~と conditional for habitual/inevitable results. あがる (get stage fright) + ~てしまう (unintended/completion) = 'end up getting nervous.' A self-aware confession of public-speaking anxiety.