Noob DM plot help

It’s my first time as a DM and I’m running the new starter set campaign with 4 players. We range from inexperienced to brand new to DnD.

There’s no plot or story in the starter set so I started to create one based on the players actions. We’ve been meeting biweekly for the last 3 months.

I’ve been giving out hints, but it’s time to make it more concrete. To what extent do I guide them to progress through the plot, or do I keep it loose. Does it matter if they “figure it out”?

They think they accidentally killed an orc when falling off a table, but the orc was already poisoned. There’s a cult who killed the orc to prevent him telling what he knows to a keep official.

Lots of secrets etc. and they are about to meet with an informer who worked for the orc.

How much do I lay out and how much do I let them figure out (or make up) on their own.

Bonus question - how do I make it more fun and engaging for new players?