Hello Justin,
Jemand is used usually as Subject, while jemanden is used in Object case.
To clarify this further, the first sentence, ICH is the Subject that's why you need to use JEMANDEN to express object case. However, the second sentence JEMAND is the Subject.
Am not sure if you have learnt the cases in German, Nominativ, Akkusative ....etc. jemand stands for Nominativ and jemanden does for Akkusativ.
Hope my answer could help you.
Cheers, Ayman