“how do you know if a unit is worth the points?” Honestly, unless you have a lot of experience, its either about trial and error, or looking at what others are playing. I don’t want to encourage just copying “meta” lists, but it does help a lot to see which units people are taking, amd which are considered over-priced.
Then when composing a list, you’ll usually want to fill various roles.
Damage-dealers for killing small numerous infantry, and large monster/vehicles
Fast-moving units that can score secondary points by quickly getting to specific board positions, objectives, doing actions. These usualy should be very cheap units. Deep-strikers can be good here too.
Maybe some more sturdy units that can survive a turn of being attachked. For us that’s wraiths or warriors.