Project Spark is a free game with no micro-transations as of Oct 5th 2015. All core content is unlocked automatically for new accounts (including creator level-locked, champion level-locked, treasure trove and pre-order props).
Prior to Oct 5th, credits were the main form of currency in Project Spark. There were a multitude of ways to get credits, but the main ones were challenges, contribution and levelling up your Creator Level.
As of Oct 5, credits have no use as a form of currency, mainly as all props became free and part of the core content.
Similar to credits, tokens, the currency used to purchase packs with real money prior to TU16, have no use as all transactions and the marketplace have been removed with TU16.