While it is true, herbicides will kill weeds, you need to explore a little
further and decide if there is another reason why your grass is mostly weeds.
     At least it is still green!

You could apply bottle after bottle or bag after bag of herbicides,
but if you don't do something to correct the underlying cause you
may be wasting your time and money.

Let's use an example.
You have goosegrass (Eusine indica) in your lawn.You spray it and it dies. Then it returns to
take over the grass that you have just re-seeded. What was accomplished?
The garden center made money on the herbicide and grass seed.
The suppliers of those products made money. You received
very little benefit from your efforts.

A solution.
The goosegrass (Eusine indica) indicates that your lawn is
probably trying to grow on compacted soil. This is a condition that
goosegrass enjoys. To correct the compaction you could aerate
the area or apply a top dressing.
   By doing this, you have eliminated the soil compaction and added   
nutrients by adding an organic amendment. Smooth the area that was just
worked, re-seed, water well, cover the seeds with wheat straw
and keep the sprouting grass from drying out.
You should now have a grassy area that should thrive with proper care
and be relatively free of weeds without applying tons of chemicals.