Hi @fotini15,
Browning leaves can be from a number of things, such as over or underwatering, insect or fungal attack or lack of or oversupply of nutrients.
It would be extremely difficult to pinpoint one issue without some further information and some images of the plant itself.
How frequently do you water the plant?
How long does the soil stay wet for?
What is it planted in? The ground? or a pot?
Can you see any insects?
Are there other signs of issues on the plant?
Any further details and images you can provide will help us narrow down the cause of the browning.
In the meantime, you might like to check out How To Grow And Care For a Lemon Tree for some general information around lemon tree care.
I will assist further when you have a chance to provide some more details.
Jacob
