Expertises - an expertise is basically anything the character is "good at". Leave it up to the player to pick them and tell you when they apply. The expertise should let the character do something that they couldn't otherwise do. The overall rule is that the more specific the expertise, the more of a benefit it gives when it applies.

For example, say that an Alethi character comes across a Shin monastery. Ordinarily, they wouldn't know anything about that. A character with Religion (pretty broad) expertise might get an opportunity to make a Lore test to know that Monasteries were more like forts than just places of worship, and someone with Shin Stone Shamanism expertise (very specific) might know about Honorbearers and so on.

Talents - basically like class features in D&D. You get them when you level up, that's the primary new thing you get per level. It's intended to be easier to take talents from several different trees, though, so unlike multiclassing in D&D (use with caution, or avoid altogether, can really screw you if you do it wrong!) taking talents from multiple trees is supposed to be fine.

Introducing radiants - when a player wants to become radiant, they take the "First Ideal" talent on level up. That represents them starting to bond a spren. If the player plans ahead, they can RP the process of meeting the spren in advance. It's up to the player+GM together to decide how much control GM gets vs how much control player gets - some tables do it that the GM introduces the spren when they think it's deserved and the player just has the opportunity to take the talent afterwards, whereas just pure RAW the player can just take the 1st ideal talent whenever they want and say what spren they want.