Hi everyone, I'm new here

so please forgive me if this question has been answered before, BUT I haven't seemed to have been able to find the answer to it -
I have just planted a young lemon tree in full sun in my front yard (which is really a front low bank). It was in its nursery bag for too long, and seems a little yellow about the leaves, also the ants seem to love it.
I suspect it has sappy pests on it that attract the ants, and would like to put some companion plants around it. I have heard good things about onions, rosemary, dill, and calendula.
I also have a very small comfrey plant that I am considering planting under the lemon tree. People seem to have success with comfrey under their fruit trees. My question is, would it be okay under a meyer lemon tree, and how far away from the base of the tree to plant it? And any of the other companion plants for that matter?
Thanks so much for your time and advice, guys. This is all totally new territory for me!