Join a class or group of other Bengali learners, or find some Bengali-speakers to befriend.
Use a English-Bengali dictionary or key to start memorizing commonly used phrases. There are many language translators and phrase books to help you start.
Find a recorded language-learning tool to help you grasp the pronunciation of the words and phrases you are using. Listen and repeat as often as possible on a daily basis.
Practice dialogues with other learners to familiarize yourself with common questions and responses. This will help your conversational skill begin to develop.
Build your vocabulary with flash cards. Preferably, put a picture of the item on one side and the Bengali word on the other so you begin to connect the visual with the Bengali word instead of the English one.
Hire a one-on-one tutor, ask a Bengali friend or set up a conversation over the web with a Bengali speaker to test your skills.