The amount of water a lemon tree needs depends a lot on the weather conditions, soil type, and stage of growth. The most critical times for supplying water are when the tree is in bloom and when fruits are developing. Lemons like to have even soil moisture, but it should be well drained.
As for pruning, lemons need little pruning, although you may occasionally need to thin out branches if they grow in too thickly. Also remove damaged or broken branches.