I look at tile yields. To a good approximation (before granaries), 1 food = 1 hammer. Add them up and call them 'production'. Commerce is a little trickier, but a reasonable rule of thumb is 2 commerce = 1 production. Assume you have all the unimproved forests you want. Those are 3P tiles, but really they're +1P tiles; you need two food for the citizen working the tile, so the net is one. There are some much better tiles available. A simple mine is +2P, twice as strong. Call a riverside mine +2.5P, half credit for the commerce.
Farmed rice is +2P (dry), +3P (next to lake/oasis), or +3.5P (next to river). Wheat and Corn are even better. Cows and Pigs are always at least +4P, very strong tiles, and riverside even better.
Your capital starts out as +4P, +8C. Growing to size two costs 22P and yields +1P for the future, if you're only growing onto a basic forest. Building a settler costs 100P and yields a +4P new city (assume the free 1C from the city offsets maintenance). Building a workboat costs 30P and yields +2P or +3P, definitely a better return. Building a worker costs 60P and returns ...
Well that depends on what tiles he improves. Pasteurizing a Cow or Pig yields +3P (switching from a 3P forest to the 6P animal). Farming an Ag resource increases its yield by +1P (dry Rice), +2P (wet Rice, dry Wheat/Corn), or +3P (wet Wheat/Corn) depending on the circumstances. Digging a Gold mine is about +3.5P (less on desert hill, more for riverside or Financial leader), though that's accepting the 2C = 1P premise. Simply mining a hill is a +1P move (switch from working a forest to a hill), or +1.5P if you gain a commerce for riverside. So the value of a worker depends on how many and which tiles he improves. But it's quite easy for it to be a large amount. If he pasteurizes a Cow and mines a riverside hill while your capital grows to size 2, you've gained +4.5P already, more than the return on a settler. And you still have the worker to do more stuff. That's a very modest example, but it suggests why worker-first is usually best. He can improve multiple tiles.
Identify your strongest tiles and maximize them.
peace,
lilnev