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.
Hope this helps! Good luck with your apartment!