Lemon trees typically thin their own fruit rather well, but if it seems a little heavy for the tree, you can snip off any excess fruit. Typically, you want to see 5-8 inches between each fruit.
 
If the lemon tree is losing leaves, this is normally a sign that it is under watered. While lemon trees are in fruit and in blossom, their water needs increase. Increase water to the tree until the fruit are firmly set, and then you can go back to your normal watering levels.