-Start learning the characters early and don't reply on Pinyin. Pinyin is an input method, not a way of writing Chinese in the English alphabet. Nobody actually "reads" Pinyin.

-Work on your pronunciation early like Learner SC said, it's quite important in Chinese. That being said, acknowledge you'll keep getting the tones wrong, it's a constant battle, and even the best learners make occasional mistakes. 

-Work on your listening comprehension a lot, it's very important. 

-Try to enjoy it. Learning Chinese isn't easy, but it should be enjoyable. Try to use things that you are interested in as much as possible because you are more likely to persist in your learning if you actually like what you are doing. 

Best of luck! 

Probably not what you are expecting to hear but Mandarin pronunciation needs a lot of work and preferably when you are a near beginner so you can get good habits ingrained. Bad pronunciation habits are difficult to change later.

Chinese is much less tolerant of poor pronunciation compared to other languages. Say it slightly wrong and a native speaker is going to have a tough time of understanding you.