Upon moving into my house I noticed the boulevard in front of my house has literally 5+ different types of plants/grass all growing together. It looks awful and I'm just curious if it's easier to tear it all up and seed it with grass or pull the non-grass plants and seed, keeping what grass is there? I've never done this before so I was just checking to see what's easiest.
On that, if it is in fact easier to pull it all up and seed it with grass, how long does grass take to start growing?
You should determine what are officially weeds or weeds to you. Is this some sort of groundcover situation that got out of hand? Was it meant to be turf? Turf can be hard to grow under trees.
If the turf is dense and looks like it would respond to feeding try that. Then treat broadleaf weeds with something like Ortho Weed-B-Gone or equivalent with 2-4-D even though you are late in the season. If you have lots of crabgrass, treat for that now.
If it is all just sparse clumps of stuff you should probably start over by tilling everything under, removing large clumps, adding ammendments and tilling again. Rake evenly.
Broadcast a quality seed mix for the type growing conditions you have (e.g. sun or shade). Top dress and roll with a half full landscape roller. Keep evenly moist until seed has germinated.
Perennial ryes will take 7-10 days to germinate but bluegrasses and fescues can take 21 or more to do their thing. So, you might want to wait until fall if you are experiencing the conditions we are here in the Midwest.
Also give consideration to planting something other than turf if it would look good in your situation. Some ground cover and a boulder would not need mowing.