It is a bit hard to tell what is going on with your tree from underneath the leaves but it certainly is having some nutrition problems and probably insect attack. I would suggest getting some Iron Chelate to correct the yellowing issue. If it improves after the first application I would probably apply again after 10-14 days. Follow the directions on the packet. Broken down manure mulch is good and a good soaking of the root system is also a good idea. Generally you don’t have to prune citrus on a yearly basis. Prune for shape or to keep fruit within reach. You can also prune out any dead or crossing branches. Prune off any branches shooting below the graft around the lower part of the tree. Spring is the best time to prune. I hope this helps.