Which lens should I Invest in first?

Such a tricky question!

The most common "starter" lens after the kit lens is the 50mm F1.8 (either the Canon or the Nikonversion depending on your camera model)

However, if you have a cropped frame and mainly shoot indoors, or you would prefer a zoom, then there are a few more options that might be better for you. 

Read this guide to starter lenses to find out more of the available options. 

Brian Steege