Do I have to kill off all of the weeds with Round-up? No.

Will this kill grass or other plants that I want to keep? Glyphosate (the generic "roundup" will kill almost anything it is applied to if it's actively growing. It doesn't know what you want to keep.

Nearly the whole acre is covered in weeds,so how do I cover such a large area? Take advantage of the opportunity to reduce the amount of lawn you have to take care of by setting up shrub areas, native grass meadow area, more trees, and flower beds.

They are more work in the beginning, but they don't need weekly mowing.

Then how do I get new grass growing? Don't, until you have a good landscape plan. Just mow the weeds short.

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Here's what I would do ... get a friend who can identify plants and figure out what you have that is truly a "weed" versus worth keeping.

Draw up a "what I want to end up with" plan ... the areas for entertaining, vegetable gardens, kids play area, etc.

Accept that you will kill yourself if you try to get it all done this summer ... and probably can't afford professionals.

Is there a house on this land? If you plan to build, don't waste any time or energy on a lawn until after the build is over.

Can you post a picture? You said it was a wooded area. What does that mean? How much shade is in there? Shade will limit your possibilities.