Not to worry; we got ya...
Pick a pot twice the size that it's in now; pot with a third potting soil mixed with a third container mix, a sixth compost or manure and a sixth sand. No fertilizeer until spring and then half rate. Check roots in June and if they are fairly solid to the bottom of the pot, repot again, same mix, twice the size. Repeat until you are having trouble moving the container, then keep that size with pruning.
Water when the soil is dry a knuckle down, give it plenty of light and come here when you have questions