Frankly, I'd hit the library, bring home a few books, and blend the
suggestions with some you'll get here. I'll add a few ideas:
1) Lawn chemicals never have been, and cannot be proven safe. This is true
for a number of reasons, some related to legislation purchased many years
ago by the chemical industry to keep anyone from looking too closely at what
they sell, and some related to the fact that the only acceptable way to
prove their safety is to test them just like medicines, on the people who
may be exposed to them. Obviously, that ain't gonna happen. Avoid them at
all costs. Anyone who tells you otherwise is getting his information only
from the labels on the chemicals.
2) Ever heard the saying "Nature abhors a vacuum"? If there's an open space
where something can grow, something will, and odds are it'll be another
weed. So, even if you *did* clear large areas of weeds using chemicals,
another weed may well end up taking over the area before you can get grass
growing. You can improve the odds in your favor by planting at the right
time, thereby giving grass a better chance. Remember that plants are in
constant competition for resources, whether that's light, water, nutrients
or simply space without damaging factors (traffic, flooding, etc). You have
to manipulate those factors to help the weaker species (grass) overtake the
stronger ones (weeds).
3) Burlap is your friend. Go to a REAL garden center and ask them why that's
true, and how to use it to your advantage when starting grass seed. Don't
balk at the cost of a large roll. It'll come in useful for many years if you
store it carefully.
4) Be patient. The only instant solution is sod. Everything else might take
2-3 seasons, unless you choose to poison your yard.