I'm the type to get nervous, so I'm bad at speaking in front of people, you know.
Literal
I [topic-って] stage-fright-type is [because-から], people [of-の] before [at-で] talk [nominalizer-の] weak-point is [explanatory-んだよ].
The topic marker って is the casual spoken variant of は — replaces は or という in very casual speech. あがり性 is a noun meaning 'tendency to get stage fright, social anxiety' — 性 as a suffix marks 'tendency, disposition.' The sentence chains cause (nervous disposition) with effect (bad at public speaking) via から. A self-deprecating social confession.