I'd like to create a trading colony, but to get it started I want to sell crops before moving on to crafting and such, what would be a good crop to plant that can (in a reasonable amout of time) grow in the beginning of a normal playthrough and sell for a lot?
It depends on which biome you live in, but I believe potatoes are good overall in terms of yield. Cotton and healroot are more valuable, but they require higher plant skill. You can also forage wild berries without having any skill, or just chop wood. Though note that crops is a pretty awful way to make money - their main purpose is food for the colony.
Devilstrand is valuable and can be researched early on, but it grows awfully slow and is usually not worth considering, unless you have hydroponic bassins. It can make for a great trade product mid-game, especially if you have a good tailor who can make clothes out of it.
If you have any colonist with decent shooting skill, hunting is a far better way to gather trade resources - allowing you to sell both the leather and the meat (though the meat spoils quick before you have a fridge). If you plan to sell the goods by sending caravans to settlements, leather is great since it doesn't weigh much.
Turning meat + crops into kibble is another option, it isn't valuable but doesn't spoil. Great for your pack animals and can also be used as emergency food for colonists (but eating it gives them bad mood).