Begin by drawing nodes to make a polygon, the last nodes needs to intersect the 1st node. Double-click to complete polygon. After you complete the polygon if you need to straighten out the lines you can use the shortcut ‘Q’.
In JOSM, use the Select tool (shortcut=S) to select objects. Be aware that you can select both nodes and edges. Often you just want to select a building or a highway by just clicking on the edge to modify the tags.
Building polygons should be drawn to overlap the base of the building. There is a noticeable difference between the base of the building and the roof for tall buildings when imagery is taken at an oblique angle. A good strategy for drawing these buildings is to trace the top of the building first, then move the polygon towards the base of the building.
You can draw a point feature in JOSM by just selecting the Draw Nodes tool and double-clicking. You can then add the necessary tags, the correct tag for a water tower is man_made=water_tower
Note that some features such as water towers, pools, and basketball courts can be drawn with either a polygon or a point.