It’s all about the soil
Having a great garden is not just about planting nice flowers and plants, it is about the soil. No matter how green your thumb is, if the soil is not healthy then anything you plant will not thrive. So how do you know if your soil is healthy? The first, and easy way, is to look at your plants and flowers. Do they look healthy? Are they looking wilted and in need of …something? If they look healthy and always give you good blossoms and fruit then your soil is probably rich in nutrients and doesn't need anything. If they always look dreary, don't flower well and suffer from diseases and blights, your soil probably needs a boost.
If you are not sure where to go from here, dig up a couple of scoops and take them to your local garden centre. Typically, the people who work there are more knowledgeable than the people who work at the big box garden sections. They should be able to look at your soil and run a couple of tests for you to let you know if you need any additives.
Finally, if you have the money, you could send your soil off to be tested at a lab. If you really want a prize show garden this might be the best route, but, the other two options can give you enough feedback to get started as well.
If your soil does need some tender care, focus on bringing it to its best conditions for a season and don't plant anything new. You could join a local garden club and get lots of input from people who share the same enthusiasm. You could hire a gardening company to come in and do the work, or, you could just slowly add fresh soil, good compost and other nutrients as required.