If your lawn is so full of weed, it would make more sense to clear it all and start from scratch. You'll get better results this way no matter what size lawn you have.

1. Use Roundup to kill everything and wait.
2. Remove the dead weed with a shovel, hoe and whateverelse.
3. Consult a local nursery to find out some of the best fertilizers for your type of soil, weather etc.
4. Till the area and mix with the fertilizers.
5. Rake, rake, rake.
6. Seed.
7. Rake.
8. Water.
9. Enjoy your new lawn in 10 days or so.
10. Maintaining a lawn takes work too, or it will look exactly like the old lawn very quickly.