Before you think about assembler, you need to sit down and think what you need to do to achieve your goal.

eg, draw a Flowchart, if you find that helps.

Then you will have to look at what features that will require on the chip, and how to use them to achieve your goal.

Only then can you start thinking about how to implement that in assembler (or any other language).

Look at the Tutorials and FAQs sections for starters;

In particular, http://www.8052.com/faqs/120112 gives links to the so-called "bible" for the 8051...