Light against dark, dark against light.
If your paintings look flat or lifeless, it could be simply a matter of increasing the contrast between adjacent tones. When I say "light against dark/dark against light", I mean "lighter than", or "darker than" - the difference in value does not have to be great to have the desired effect. Form, space and depth are registered by the eye through variations in light and shade, so too the painter uses variations in value/tone/light and shade to imply the existence of the third dimension on the flat 2 dimensional surface of the painting.
If you have a digital camera you can readily check whether the tonal values are working in your painting: take a photo in black and white, if it makes sense in terms of form, space and depth, they work!
In Watercolour, Less is more, both in the painting and in the materials.
You do not need many materials to paint good watercolours. All you need is actually one fairly large or medium sized sable brush with a good tip, three to six tubes of paint, a palette to mix on, watercolour paper (preferably made of 100% cotton), water and towel. Keep it simple, do not encumber yourself with too much choice.