Explore widely, from water flow, ginkgo leaves, insects, animals, human figures. Try to observe and understand the object first, proportion, the mass or the volume, subtle surface transition, form or the movement, then draw based your understanding (sketch without actually looking at what you are drawing), or further more, explore in the context of design. Natural forms are the best to start with. No area of design, sketching, ideation or imagination seems to be off limits.
Sketch as often as you can, and enjoy it as much as you can! Take full advantage of the learning resources offered by the objects around you. Without having to worry about designing anything, you will be able to study perspective, form, lighting and shading, reflection and color. There more you practice, the wider range of objects you are familiar with, the better your drawing will naturally become.
Create Your Own Visual Library! In your daily practice, make sure you have a huge bank of knowledge to draw from, regardless of the given design problems.