I'm still new to the hobby, but by now I've painted over two hundred miniatures from 2 cm tall to 20 cm. I've made lots of mistakes but I've learned to avoid some and fix others. And yet I still experience a little trepidation when faced with a new miniature. I guess it's that paralysis you hear about when an author is faced with a blank page, or an illustrator faces a blank canvas. Where do you begin to paint? What colours do you need? What if you mess it up? I've thought about this phenomenon, and I think I've come up with some things to help.
Before you even start to think about having to paint a miniature, stop. First thing's first: look at the miniature. Hold it under a magnifying glass so you can see the finest details. Look at it the way you'd look at a Rodin sculpture in a museum. You don't have to analyse the miniature the way an art student might, but I find that the better I feel I know a sculpt, the easier it is to imagine painting it.
Try looking at the miniature in the abstract, too. When I buy a new miniature, I've invariably bought it because of what it signifies in the game I'm going to use it in. It's natural for me to look at an unpainted miniature as what it represents. I look at it, and I imagine it rampaging through the streets, or galloping across an open field with great castles in the background, or flying through the emptiness of space amid laser fire. It's exciting, and that's why I bought it, but the problem is that it's too exciting. You aren't physically painting an idea, you're painting a hunk of plastic. So look at the miniature as a sculpture, as something that's been designed, as a collection of components that form a larger object. Look at details, like where boots end and greaves begin, how a robe flows under a strap, how a sword is tucked behind a shield or a backpack, and so on.
But most of all, just get familiar with the miniature. The more comfortable you are with the miniature, the easier it is for you to conceptualise how to paint it.
I have no fashion sense in real life or in game life. Black on black is my default setting in real life, and while that's appropriate for some miniatures it's also pretty boring to look at. The problem is, how do you know what colours to use for a miniature when you also have no idea what colours to use for yourself? I have two ways of solving this conundrum.
First of all, there's box art and the Internet. When in doubt, just look at the box or look for your miniature online and see what somebody else has done. I do this a lot.
My other solution is to move from broad colour definitions to specific. First, I try to categorise my "vision" for the miniature in the hues of the rainbow: Red, Orange, Yellow, Green, Blue, Purple, and Black or White. I don't think about the shades of those hues, I just think in broad terms. Do I see this miniature as a red miniature, or a blue miniature, or Green? Once I have a general colour category, then I start thinking about the specific shade, based on the paints I either have or am willing to purchase.
In both cases, I'm thinking as if the miniature were going to be monochromatic. If I could use only one colour for the entire miniature, what would that colour be? I'm not thinking about fleshtones or highlight colours, I'm just trying to find the dominant colour that wants to embodies that particular miniature.
Finally, I look at the miniature for parts that need to be painted with colour that's not my dominant colour. There are some obvious things that I more or less mentally skip, like flesh and cybernetic limbs, plate armour, scabbards, leather pouches, and so on. I know what colours those are going to be, so much so that I have pots of paint labelled specifically for those items. Those are good to identify visually, but you probably don't really need to pick out colours for them.
Components you want to find at this stage are the "non-standard" things that are specific to the miniature. For instance, on my Roman army I know that scabbards are brown, plate armour is grey, skin is a fleshtone, and so on, but what about the tunic? The soldier's tunic is a specific element that requires consideration based on culture, history and, pragmatically, game mechanics. On an Adeptus Mechanicus miniature, the list of what's "obvious" is different, so the components to mentally skip differs depending on what "normal" is for you.
The first step of the first mini can be intimidating, but once you start you're likely to open the figurative flood gate (especially if you're painting a whole army). It's often the case that the first miniature you paint establishes a template for the rest of the miniatures even vaguely similar to it, so take the time to get comfortable with the miniatures you need to get done. Then start painting!
Photo by Seth Kenlon, Creative Commons cc0.