Start with pronunciation and tones
Starting out with pronunciation and tones is so important because once you have adopted a certain style of speaking, it will be hard to change that later. So it is better to learn correctly from the very beginning. This is the part where you probably want a native speaker helping you and pointing out your flaws. Especially the tones are hard to distinguish for the untrained ear, but they are crucial in Chinese.

Learn Chinese Characters with an intelligent method
While you are getting confident with pronunciations and tones, you should also start diving into Chinese characters. In my opinion, there is little sense in learning Chinese without also doing some rudimentary studies in Chinese characters. For starters, you do want to be able to find the bathroom, read the menu or find your train if you end up in China. But more importantly, characters will give you a whole other level of understanding about the language and improve your speaking and your listening as well.

Stay focused and practice!
The only way to ensure success is still to stay focused and to have realistic but challenging goals you want to reach. The HSK tests might be one, traveling to China might be one, converse with your business partner in Mandarin might be another. Whatever it is, with that goal in mind, keep practicing and revising. As someone said, the moment you stop learning is the moment you start dying. This is especially true with languages: Without continued revision, acquiring a second or third language and retaining it is near impossible.