My lemon tree is full of fruit ,leaves are green with yellow marks on them,doesn't look healthy.What to do?

This will happen, - if the Tree needs feeding, you may find it needs an acid type of Feed +Soil - also where you are keeping the potted tree usually better at this time of year under cover eg greenhouse, also check the leaves for bug infections.

Magnesium deficiency more than likely

Treat your lemon tree like an Olympic athlete - feed well and regularly, hydrate well and keep diseases at bay. Trim out dead wood and if leaves are yellow, mix some Epsom salts into your citrus fertiliser. When you mulch, stay away from the base of the trunk - citrus are prone to collar rot.

To treat, remove all infected leaves and dropped fruit from the ground. Prune the lower branches from the tree, those that are more than 2 feet from the ground. Then spray with a fungicide such as Agri-Fos orCaptan. Botrytis fungus - Botrytis rot is yet anotherfungal infection which may afflict lemon trees.10 Jan 2016

Lemon Tree Problems: Treating Common Lemon Tree Diseases

https://www.gardeningknowhow.com/edible/.../lemons/common-lemon-tree-diseases.ht.. What is the Treatment for diseases of lemon trees Google Search

short on a mineral in the soil
read google pages on citrus fruit problems
could be lack of iron or manganese mineral

Lemon trees are extremely sensitive to over watering, only thing you can do is make sure you have good drainage. Dig channels at low points to make sure ground water moves away. Just a small point, you posted your question on the Cars & Trucks section, you may be missing some better solutions by posting here, however, I've raised a few lemon trees in Spain and in my front room in London so I have a little experience in this area.
Good luck

Weather? Cold? Try heavy mulch at the base.

It could take up to 7 years in some climates to fruit. It's self pollenating but likes a Ph level of 6 to 7.5. Get a soil Ph tester & check your soils Ph levels and maybe fertilize it with a fruit tree fertilizer. That's the best I can tell ya. Hope it helps