What is the IUPAC (systematic) nomenclature for the following compound CH3CH=CCH2CH3 / CH3

Your structure doesn't show up correctly. I'll bet you could show it as: CH3CH=C(CH3)CH2CH3. As with all organic names, you want to show:
a) the longest carbon chain. [here there are 5 carbons]
b) the dominant functional group [this is an alkENE] and its location [here it's on carbon number 2]
c) any other substituents [you have a Methyl group] and its locaton [here it's on carbon number 3]
Put it all together and tell us what you get. (If there is sufficient information, you could also designate the substituents as Z or E)