you’re not a planner (I’m certainly not), don’t force yourself. You can always find something worthwhile by taking a stroll or by asking at the hotel.
If you’d like to experience a top or very popular restaurant, it’s of course advisable to reserve several weeks or more in advance. Lunch reservations in such restaurants are typically easier to come by than dinner reservations. If you’ve got your appetite set on a top restaurant, then it’s likely that you’re staying in a luxury hotel, in which case your concierge will be the best bet for getting the reservation. Having said that, one way to snag an elusive table is to call the day you wish to go there or the day before. Since top restaurants require confirmation the day before (sometimes by noon for an evening reservation) you may luck into a recently cancelled or unconfirmed reservation. There’s no harm in trying.
If you have your appetite set on any other particular restaurant you should at least call (or have the hotel call) before showing up. If you pass by a restaurant of interest during a touring day you might stop by and reserve then. Most bistros, brasseries, and regional restaurants will be able to accommodate you with little or no advance notice.
Lunch is typically served noon to 2 p.m., though you may stay seated later. Aside from brasseries, some bistros, and very touristy or fast food places, restaurants rarely open for dinner before 7:30, and typically not until 8 p.m.
(If you are invited to a Parisians home for dinner the invitation will usually be for about 8. Don’t arrive early unless you wish to see your hosts in a panic to get ready. Arrive 10 minutes after the appointed hour and you’ll find them cool as cucumbers. Dinner is served after the aperitif, and only a nervous host rushes the aperitif.)
The time of the last order in a restaurant, meaning the latest you can arrive or reserve, varies from place to place and often depends on the type of restaurant. In Paris most restaurants will accept new arrivals until 10/10:30 p.m., perhaps later. (In small-town France you may be out of luck after 9:30 p.m.) You can then generally stay seated well past midnight. There is a natural tendency for thriving restaurants to funnel reservations into two dinner seatings, one beginning 8-8:30 p.m., the other 10-10:30 p.m. Citadels of high gastronomy typically have a single seating.