、。
If you want to hear a scary story, I'll tell you about a dream I had a few weeks ago.
Literal
Scary story [subject-が] hear-want [conditional-なら], several-weeks-ago [target-に] I [subject-が] saw dream [genitive-の] thing [object-を] tell-give [emphasis-よ].
Two interesting constructions. ~なら is the topical conditional — 'if it's the case that you want to hear,' as opposed to ~たら which would emphasize the temporal sequence. 夢を見る is the standard idiom for 'to have a dream' (literally 'to see a dream'). And 話してあげる adds the benefactive ~てあげる, marking the speaker's offer as something done for the listener's benefit. Inside the relative clause 私が見た夢, が marks the in-clause subject.