All you have to do to get a task is click on Talk-to Slayer Master and she will give you a task. If your task is too hard you can ask her for a new one, or if you do not like your task you can type ::resettask. You can buy items from the slayer master using Slayer points or Boss points. Both shops have different items to help further your combat skills. In order to receive boss points you will need to be the maximum level of slayer, which would be 99. The slayer master will give you Boss slayer tasks and you will have to kill the boss that she gives you. You also have the ability to reset your task or get an easier task.