I started my fist mission, not quest. I don't know how to finish it, its telling me to help farmers plant seeds in some field but I can't find it. What am I supposed to do?

It's a bit tough to explain but missions do not have to be done. Somewhere on the mission sheet it says the duration of the mission and the primary stat for it. All you have to do is select a pet of the missions minimum level (also listed) and which is proficient in that stat and click the green button. For the duration listed the pet "is on the mission" and you cannot do anything with it. However when it returns, if it's stat is high enough, you get gold, experience for the pet, reputation with the town you accepted the mission from, and mission satchels which give you goodies such as more gold, potions/elixirs and gear. All you do is send the pet and then wait. Hope this is clear enough.

Also it is a good idea to have mission pets (pets with all the level ups and profficiancies dumped into one stat, used for missions) since that leaves the party pets free and has a better chance of success in missions since those pets have a high stat,

Here is a guide that you may find helpful