Crops can be found and grown on any Creativerse game-world.
Crisphead Lettuce, Turnips and Horned Melons grow over time in specific Biomes, so the longer you play, the more of these you will discover. Wheat however can only be grown by players from Wheat Seeds obtained from Tallgrass or Savannah Tallgrass.
Currently, only four kinds of cultivable Crops exist:
Tree Saplings can also be grown (and fertilized), but not consumed nor cooked; and they also do not need tilled soil, just green Grass. Queen Bees can be grown into Beeswax that is also not consumable.
Crisphead Lettuce can most often be found in Forests, and Turnips most often in and Horned Melons only on Jungle floors. While these Crops are in fact able to spawn on suitable blocks like green Grass, Dead Grass or Detritus in other Biomes as well, this is very random and such player-made artificial biomes might not always be suitable to let them spawn.
Wheat Seeds can be obtained randomly by collecting Tallgrass or Savannah Tallgrass (even after placing it yourself). As usual, harvesting these long grass stalks can be accomplished by using the Arctek gauntlet to "pull" the Tallgrass (with or without any Power Cells) equipped.
Seeds can only be planted on "tilled land". You have to use the Plow on either Grass, Dirt or Mud to transform it into tilled land. All seeds will also need Water, Bog Water (only sparely!) or Mineral Water 1 block close to the block of tilled land in order to grow. The liquid can be placed directly underneath the tilled land as well.
Seeds (and seedlings) can be fertilized with Pigsy Droppings to grow faster, but this is not necessary to grow Crops at all.
Please note that Seeds will stay fallow in non-suitable areas. For example if the environment is too cold (snowy or icy, mainly wherever Water turns to Ice), too high (above an altitude of ca. 150) or too hot (Lava layer) as well as in Swamplands or too close to larger pools of Bog Water. Seeds do not need any light to grow, so they can be planted on blocks placed into buildings or underground too in any Layer except for the Lava layer. You'll just have make sure that Water or Mineral Water is touching each block of tilled land.
Simply look at the Seeds to see if they are fallow (it's written in red right above the name of the Seeds) or "growing". You cannot fertilize fallow Seeds, so don't worry about wasting fertilizer this way.
As long as the Seeds are not fallow, you'll just have to wait until the seeds will grow into seedlings, sprouts and plants - only when ripe or grown you can harvest them. Wheat will need approx. 50 minutes (real-life-time) to grow from seeds into ripe plants, even if you're offline. Lettuce needs about 60 minutes, Turnips not much longer (ca. 61 minutes), Horned Melons need ca. 90 minutes until they're ripe.
You can harvest Crops as soon as they are "ripe" (and sparkling) by pulling them with left-click with your gauntlet like you take/mine any other block in Creativerse. You might want to de-equip your Power Cell first so you won't accidentally pull the tilled land too. If you wait longer (ca. 24 hours of real-life-time), then the crops will turn into "Grown" crops, and will give a little less harvest than during their optimal "ripe"-stage. They will never whither.
"The design for crops is that they'll progress through a few states before you can harvest them, the last two states are Ripe, and then Grown. Ripe giving you the best return, and grown giving you less of a return but still giving you the final product (unlike harvesting a sprout, which only gives you seed). You have a little over 24 hours after a crop reaches the "Ripe" state to harvest it before it becomes "Grown". It's essentially the same thing as the crop withering, but with less of a punishment." - Thor 10-23-2015 See forum discussions
Don't wait too long with planting Seeds on the tilled land again, or put anything else on it, otherwise all the tilled blocks will revert into Dirt blocks within a short period of time (randomly between seconds to several minutes).