Water-based lubricants
The most versatile lube: compatible with all toys and condoms, doesn't stain and washes off with water. If you only get one, make it this.
When to use it
Whenever you like: with silicone toys, with condoms, solo or to ease dryness. Using lube is about comfort, not a problem.
What to look for
Glycerin- and paraben-free if you have sensitive skin or are prone to infections; gel textures last longer. For anal play choose a thicker one: see anal lubricants. All types in lubricants.