I need to create requirements model for my software(first step of software engineering), But I don't know which icon of UML can help me to implement requirements model.
UML is a complete modelling language, with lots of diagrams.
It is not totally clear what kind of requirements you want to represent: is it about the structure of your system ? Or is it its behavior ? Without further details, it'll be difficult to give the right answer.
Nevertheless, if you need to start somewhere, usually you would start with the use case diagram: it shows the system and the expected functions that it shall offer to the actors (users, other systems) in the environment. So, this model gives a clear picture of the main requirements: what the system shall do for which actor.
Remark: You mention an icon. I suppose you mean the icon in a modelling software. This will of course depend on the software. Usually it's some kind of stick-person with an oval.