beginner growing outside need some help

Yeah im new to growing pot, i already germinated the seeds and they are in the ground already. just wanna make sure im doing things right, i used cow manure mix, garden soil and top soil and a little humus is this mixure okay to use? i water every couple days but some of the plants seem like they are not growing and some leaves are yellow. they have been in the ground for about 3 weeks. now how do you know how much fert to use i heard things about rapid gro but im not sure if i really need fert i dont wanna kill them if somebody can help me id really appreciate it!