Can spells be activated like instants are in magic.

For instance if my monster attacks and gets blocked by my opponent's card, can I activate Creeping Plague then or do I have to before I attack.

No, spells can only be played during your "Summon creatures and cast spells" phase with the exception of Shield Triggers which get used as soon as they are broken as shields

which get used as soon as they are broken as shields if you choose to cast them for free at that time (otherwise you put the spell in your hand as normal)