You can use the mana produced by Myr Reservoir’s first ability to pay an alternative cost or additional cost incurred while casting a Myr spell. It’s not limited to just that spell’s mana cost.

The mana can’t be spent to activate activated abilities of Myr sources that aren’t on the battlefield.

A card, spell, or permanent is a Myr only if it has the subtype Myr, regardless of its name. For example, Myr Reservoir itself is not a Myr.