Hi donkey guy,
Re your plum tree.
First it will pay you to put a cane into the pot to tie your tree to as it grows. This will keep it straight.
The tree needs to go into a dormant period, so at some point you will have to move it outdoors. If you have severe weather then protect it with a covour of fleece especially round the tub as root damage can happen if the roots freeze solid.
In the spring, once it has broken into growth tip the tree back four or so buds, this will do two things. It will strengthen the trunk and it will force the tree to start to produce some side shoots.....these will eventually become the trees lower branches. Do not cut the leading shoot back too much or the new branches will be too low.