I'm new here, and was hoping someone will give me guidance.

I live in Washington, D.C. and have been growing a Meyer lemon tree in a large, West-facing window for about six months. All things considered, everything is going well. But I want to make sure the tree grows in a healthy way.

I bought the tree from a hardware store in March. The clerk could not tell me how old it was at the time. I'd estimate the size was 1 foot tall by 1 foot wide.

I have no yard or outdoor living space, and thought I'd see how long the tree would last indoors. Since I bought it, the leaves have multiplied, the branches have grown several inches, and I now have 5 in-progress lemons that are green and about the size of golf balls, or slightly larger. There are also several sharp shoots sticking out of every branch (but not at the base of the tree).

From time to time, some of the leaves have turned yellow and fallen off. But this happens infrequently and has never affected the whole tree. Overall, the leaves are a lighter green than I have seen on other Meyer lemon trees (in photos), but I have attributed that to the tree's location indoors.

I lightly water the tree once a week. Nothing special.

I'm wondering:

-When should I move the tree out of its black, plastic nursery pot and into something larger? What soil should I use? What size pot?

-Can I simply 'pot up' at first? Or should I be prepared to prune the roots, etc.? I have never done that before.

-Is it normal for the lemons to be at golf-ball stage in September? Is it a problem if my tree is off schedule?

-What else can I do to ensure the health of my tree? I read about checking the soil's pH level, watering with vinegar, etc. Do I need to prune it?

Thank you in advance ....