Elizabeth has mentioned some key points such as tongue and lips position, to which I would add the way you use your jaw and even how you breathe (for certain sounds). You literally have to retrain your articulatory system if you wish to articulate sounds clearly and accurately, though this is something many people are too lazy to even try. Along with intonation, stress (where you put the emphasis in a word or sentence), rhythm and connected speech also come into play once you reach a certain stage - it will make you sound more natural and will help you a great deal when it comes to understanding native English speakers.
At the end of the day, it all depends on how much effort you are willing to put in, your natural ability, your love for the language and also your mother tongue. I can see yours is Spanish, as is mine. Just to give you an example, there are 5 vowel sounds in Spanish and no fewer than 12 in English, so you get the idea.