I will add a couple of more things....(actually it's more elaborating on what others have said). Take your time. One of the fun things about building is putting it together. We get that excitement when things start looking like what's on the box cover (minus painting) and taking shape, and it is easy to just slap everything together. When you paint it, you see errors. Mismatched joints, places where filler should have been put in or just things that make a potentially nice model into a mediocre one.
Also, there is the quality of the model manufacturer. Tamiya, is one of the best, and longest manufacturers out there. Very little filling is needed on most of their joints and panel lines are always recessed. Detail is important. Many manufacturers have raised panel lines. For a child, or a beginner modeller these are fine, but if you start out with a quality model, and take your time, you will be rewarded for your patience.
Get a half decent airbrush. They're not hard use, and the biggest issue is taking them apart and cleaning them afterwards. Painting by brush will still be needed for smaller parts, but for the large areas, you definitely will see brush strokes and generally the paint goes on thicker which obscures details.
Aspire for greatness. You will be surprised at what you can do, if you try. Treat every joint on a model as a challenge to fill and ensure the details remain sharp.
In short, do your best. You will notice over the years, once your display cabinet fills up, on how the quality of your builds improve.
TazMan2000