Tips please for smoothing an unsupported surface

Hey people! I'm completely new to 3d printing and have been teaching myself for the past couple of weeks since purchasing my first printer - The Ender 3. I've learnt a bucket-load already but still have along ways to go. Anyway what I was hoping was if someone could help with is how to get around the printer printing the top level of some of my models all bumpy. I assume it's because there's nothing underneath to catch the filament as it comes out so it kinda just hangs there and doesn't smoosh together to make a nice, smooth surface. Sorry if this is confusing.... Here's a pic that explains it better.

Attachment 12308

How do I fix this? Also if, it's any help, I'm using Cura 3.6, it came with the printer so I'm not sure if it's any good or not. A friend recommended using a solvent to smooth it out, would this work and if so, what do you recommend? I'd prefer not to resort to that as I'd like to learn how to print things perfectly in one go.