The most common mistakes newbs make are growing in inadequate soil, not having enough light or ventilation, and under/over watering. If you can correct these 3 issues your plants will stay healthy and thrive every time.
The strain you are growing also makes a big difference. Most indoor growers prefer indica dominant plants because they stay shorter, are easier to trim, finish in less time and have denser flowers. Outdoors you don't have to worry about vertical space so sativas are the best approach, they take longer to finish but are resistant to mould and insects, are more forgiving of climate change, and are more potent.