Thanks so much for posting! I can’t recommend making moodboards enough, so I’m so excited you’re prepping with one as you take on a new space! I use Photoshop for mine and that has worked very well for me, though I’m sure there are many other similar programs.
There isn’t a specific item I will search for first overall, because each room is different! Some rooms come with furniture I need to consider in the design and others are completely bare! But I would recommend searching for the most size-imperative pieces first. If you have a smaller room or specific area that you need *just* the right size piece of furniture for, it is a lot easier to get that piece first and fill in around it, than to get, say, side tables first and realize you didn’t leave enough room for the sofa.