Be critical of your efforts. If you think you've got a perspective problem refer to a good reference book on perspective . Learn about tonal contrast, and light and shadow and apply it to your sketches using softer grades of pencils, or water-soluble pencils in shades of grey, which really enhance a sketch. A very helpful book to help you understand tone is Light in Watercolour by Lucy Willis. Good design is paramount to the success of your painting.
Poor composition can ruin an otherwise worthy picture.
But take heart - the rules aren't too hard to learn, and the learning can be fun. Remember - you don't need to know everything all at once to start making pictures. Just learn as you go, learn from your mistakes and count them as stepping stones on the road to success. A detailed and practical guide for beginners and experienced artists. If you hope to do still lifes, you'll still need a reasonable knowledge of perspective and what happens to ellipses when viewed from various angles.