Research programs. Consider their convenience, cost, reputation, curriculum, style of instruction and specializations offered.
Apply to the programs that interest you. Depending on the program you will submit a transcript, letter of recommendation, test scores (particularly if you are not a native English speaker) and a personal statement or essay.
Choose a program by comparing its cost, in time and money, and its benefits.
Start taking courses to fulfill degree requirements. Work with an advisor at the school who can help you achieve your goals.
Apply what you learn in your current job. The more you can apply as you go along, the better you will retain what you learn. This is especially important since you are learning online, which lacks some of the interactive components that are so important in business.
Look to the future. What are the next steps that you will take once you earn your MBA? With an answer to this question you can thoughtfully plan your program of study.