Do we have to use any article before job title?

In just about every case that comes to mind, you would use a definite or indefinite article when describing your employment. The reason for this is that the job title (such as "Engineer", "Farmer" or "Accountant") is a noun and should be treated as such in a sentence.

In this situation, the correct sentence would be "I am a Senior IT Engineer" (one of many Senior IT Engineers) or "I am the Senior IT Engineer" (you are the only Senior IT Engineer).

In the same way that you wouldn't say "I am person" but rather "I am a person", saying "I am Senior IT Engineer" is incorrect.

I'd say the main difference is whether there is more than one person in that position. So, for example, if you were to say "I am Senior IT Engineer" you would essentially be saying "I am the Senior IT Engineer," implying that you are the only one who holds the title. (Personally I would go with "I'm Senior IT Engineer," since "I am Senior IT Engineer" sounds sort of robotic.) If there is more than one Senior IT Engineer, then you should always say "I am a Senior IT Engineer."