<p>I have an object that I want to print in 3D. But I have a few questions about it. What are the things that I have to watch out for when 3D printing?  </p>

<p>I know how to change the metric size etc. Some  people said that it's best to set the thickness to a low amount and not make the object solid (to leave the inside empty/hollow) in order to save money when printing.</p>

<p>Is this true or does it not matter?</p>

<p>Also what if I want two parts of an object to be separate colours or materials?  Do I have to change this in Blender? </p>

<p>Any advice and information would be helpful, thanks.</p>

<p><a href="https://i.stack.imgur.com/TG6Mg.png" rel="noreferrer"><img src="https://i.stack.imgur.com/TG6Mg.png" alt="enter image description here"></a></p>

<p><a href="https://i.stack.imgur.com/KIEy1.png" rel="noreferrer"><img src="https://i.stack.imgur.com/KIEy1.png" alt="enter image description here"></a></p>

You are correct about the walls. Using a
Solidify
object modifier is probably your best bet. A low
Thickness:
value (
0.1
is probably good) helps keep the walls thin but strong. You can monitor the thickness while you adjust the value from
Wireframe
view.
Additionally, and
this is probably the most important thing to know
, your mesh must be clean. By clean, I mean it must all be one piece. No separate cubes, cylinders, etc. that you added while modeling, just one solid piece. Think about it this way. If you have added a cube and part of that cube is inside the rest, it might look good from the outside. But the 3D Printer isn't printing the outside, it's printing everything. So that wall, albeit hidden, that is present on the inside of your mesh
will be printed
.
Bad:
Good:
Lastly, if you have parts of your mesh that can't be printed from the bottom up, or wouldn't stand by itself, consider adding supports. You can always cut these off later.
Leg added because it wouldn't stand by itself: