Both can be used. I start work tomorrow. I start work in an hour. I start working tomorrow. I start working in an hour. 'Working' is the gerund of the verb 'to work', and so is used as a noun or the object of a verb (in this case, 'working' is the object of the verb 'to start'). Likewise, 'work in the first example is also used as a verb object, so they are interchangeable in this case.

Give us more information about the context and so people can help. Both are correct and it depends on how you use it. for example I start work at 7:00am. I will start working right away.