1) Buy a good broadcast fertilizer spreader- not the kind that drops the stuff directly down, but one with a spreading wheel that is turned by walking forward.
2) Buy a crabgrass killer from your local garden store. Apply it asap, then apply it again in a week to spots where the crabgrass has not died.
3) Aerate- you can do this yourself by renting an aerator or hire it out. It's a pretty important step, so don't skip it.
4) Overseed with a mixture of seed that matches your conditions. Tell your garden store what your yard is like (sunny, shady, etc.) and they will help match the seed to the conditions.
5) Water it in and keep watering until the grass looks like it's healthy and established.
6) If you have patches where this is no grass whatsoever, consider renting a slit seeder (below). These babies do a great job.
YouTube - Slit Seeding by Steven.
As for the question about weeds, I only recommend using Roundup when you want to kill everything. If you have patchy weeds throughout the yard, using Roundup on each one will result in a circle of dead plants (both weed and grass) at each spray point. Instead, you can use a broadleaf herbicide (2, 4-D is the most common)
You may find a crabgrass killer that also kills weeds (including the clover). If you don't, then look for something that is both a fertilizer and a broadleaf herbicide. Apply it now, then apply the crabgrass killer.
Do you need a professional service to maintain your lawn? Well, you might want to ask a landscaper to renovate your lawn, but I have never seen a real reason to have someone come out and treat a homeowners lawn unless the homeowner hates the work. It's not hard to fertilize a few times a year. My own opinion about professional services is that they use too much nitrogen and too much herbicide. I prefer to treat for weeds only occasionally and fertilize twice a year to keep lawns healthy, which often keeps weeds from growing (but I also have a higher tolerance for weeds than many homeowners- I just don't like throwing chemicals on the grass my kids play on). The best thing you can do for your lawn, once established is to keep it watered, fertilize and then aerate and top dress in the fall. All these things can be hired out, of course.