Just write. Write and read. It sounds silly, and even a bit redundant but continuing to write as often as possible helps tremendously. I'm a bit of a perfectionist, so when I first started writing I didn't get very far very fast because I was never happy with my first few sentences and kept "redoing" it over and over. Your first few sentences and paragraphs when you start writing are probably going to be horrible, just ignore it and keep going. You can always edit later! Once you get into a rhythm, conversations and descriptions flow more easily.
Second, read. Read, read, read. It inspires new ideas, refines old ones, and helps you keep your grammar and punctuation accurate. I decided not to reinvent physics for my book, which meant that I had better understand how nature, politics, and relationships worked if I wanted anything to be believable.