I am by no means an expert on this, but you could write 30 pages on just what seeds to plant!! That being said my mantra is to keep things a simple as possible. Also, I didn't mention it above but I HIGHLY recommend buying the Food plot book from QDMA. It is a great guide and talks a lot about looking at your property and determining objectives way before you decide on planning for food plots. Make sure you look at your land topography, drainage, intrusion from neighbors if applicable, travel areas, human foot traffic etc, etc. Also, make sure when you decide on a location, that you look at your own personal travel roads/trails. This is important b/c if you have one of your main trails next to a plot (extremely common b/c that's how you get your equipment to the plot), you are going to be bumping deer out of the plot on the way to your hunting spot. Although they will still use the plot, they will do it at night well after dark.
Also, I've seen it many times when guys will plant a food plot in an area that was previously the best bedding area on their property! They cut the grasses and shrubs b/c it's a good plot location and devoid of large trees (i.e they do not have to cut trees and pull stumps in a wooded area of their property for plot preparation) only to disrupt one of their only or best bedding/cover areas on their land. Sure the deer will hit the plot, but when it's grazed over, those deer are gone in the offseason b/c they have lost their bedding area.
Now, on to the seeds......
What to plant depends on your objectives (kill plot vs. true destination plot as a spring/summer/fall food source, protein for antler growth, vs. a winter food source to keep deer on your property year round, etc, etc).
Although there are 50,000 different ways to do this, my recommendation would be to start with a perennial such as chicory/clover or an annual such as brassicas. Many seed companies sell these blends, Imperial Clover, Biologics, Pennington, etc. etc. but you can also get seed blends from Agway. I think the Imperial Clover or Fusion is a great clover seed for early planting that will take you into the cold season as well. It's a great food source and pretty easy to grow, but needs some TLC like mowing once it really gets established. One of the LOU members posted pics this year of his clover plot and it was truly spectacular to see his pre and post mowing pics!! Probably the most lush I've ever seen. Clover may also take a season or to to "get going" but it's relatively hardy and can tolerate different growing conditions. The first year you plant it'll grow for sure,but it's likely to take a year or two to get super lush and thick. depending on how much rain you get during the growing season. It is often incorporated into mixtures of other seeds such as cereal rye, chicory, wheat.
You can also plant brassica mixtures later in the spring/early summer for a good food source later in fall and especially winter. It's an annual and will need to be replanted yearly, but the deer will absolutely tear up the turnips and radishes under ground after the first snow fall. They will graze freely on the greens throughout the summer as well, so it can be frustrating b/c if you have high grazing pressure as they will eat all the greens before the turnips/radishes can get established. Eventually the tubers get sweet and the deer absolutely love them later in the season! I have also experimented with planting cereal rye first thing in the spring as more of a green fertilizer and re-discing it after a month of growth before it gets too high and then planting brassicas seeds after that. This is cereal rye and NOT rye grass. The deer eat the rye too so it provides a good food source and a green fertilizer to the plot.
Other folks will plant corn, soybeans, peas, rape, etc etc but I just don't have my own personal experience with these seeds. Hopefully some other folks can chime in on their experience.
Keep it simple and have fun.