I just cleared an area of all the leaves and underbrush on some land I have. It will be a small plot but the ground is still frozen. when should I start hitting it with round-up, fertilizing and dropping seed?
Depends on what you plan to plant. Seeds such as clover and alfalfa can be "frost seeded", meaning that you can actually spread the seed when there is still snow on the ground. The idea is that as they ground thaws, the snow will give way to soft muddy dirt which will make good contact with the seed and begin germination.
You shouldn't spray any Roundup until it warms up and the grass starts growing. The way Roundup works is that it take the poison in through the leaf and takes it all the way to the root. This is what kills the plant. If it isn't growing yet, Roundup won't work.
I would wait until mid/late April before thinking about spraying, then wait a week or so after you spray. Then put down your lime/fertilizer. Rake it (work it) in with the soil if you can. Then spread your seed. Don't forget to press the seed into the soil by walking or driving on it.
If you are going to try corn, sun flowers, or soy beans you should wait until late May or early June. They really need warm temps or the crop will be a failure. Oats can be planted in early August and make a great fall plot. Buck Forage Oats work pretty well and can last into the colder months.