- Great Question!
- Tough to answer for sure.
- Depends on both the needs of the Imperium and our planet's determined specialty...
- Definitely manpower in the beginning.
- Agri resources would be a close second.
- Our classification as a paradise world would not be far fetched.
I have done quite a bit of research on this type of topic myself. As you know, tithes can be in the form of just about anything: ammunition, vehicles, clothing/armor, manpower, etc. For your question specifically I think it would be easiest to consider what would happen if the Imperium entered orbit and brought us into the folds of the Imperium today. Let's also say that we either knew we were hopelessly outgunned and surrendered or that we were completely taken by surprise and had no time to react and mount a defense; therefore all of our infrastructure is still intact.
So first I would see the Adeptus Ministorum come in and quickly work to convince us that the Emperor was here before and that the reason we have so many different religions is because they are all based upon the Imperial Cult and that the Emperor is the true God we have all been believing in for so long... and that this is the beginning of the second coming (maybe even that the second coming is actually Roboute). I would see this swaying a great many people and once we find out the evils are all the xenos and forces of Chaos we would all bond against our new enemy and fall right in line. We "Earthlings" love the ability to unite against something and religion has always provided the best way to get us to go to war for something bigger than ourselves.
Then, the Imperium would acknowledge that we have way too many people for this planet and start up a few regiments of Imperial Guard. Beyond giving our standing armies las-rifles and such I would also see them upgrade some of our current armed forces, such as putting las-cannons and krak missiles on tanks, helicopters, and planes. The bulk of what we have though would be incorporated since it both exists and is known to us... and more importantly sending all of our "military might" (nukes, planes, tanks) would provide relief to the idea that an uprising would occur.
Then we would send those armies to whatever battlefield needed men and see what happens with both our conventional weaponry and tactics as well as the incorporation of the newer advancements. Depending on how well we do in combat and how well our own conventional arms do we might see the possibility of forging more, but that is doubtful given many of the Imperiums arms are just upgraded versions of our own...
Next I would see them looking to determine our planet's best contribution to the Imperium. First they would begin noticing how green our world is and how about 40 percent of land is agricultural making them possibly lean towards an agri world. Once you pull out a bunch of people and tear down some stuff it opens up the possibility for a lot more farming to be created.
I am also confident they would look to make Earth a paradise world at the same time. At first thought, we have a lot of beaches, but we have so much more. We have skiing, hand gliding, Hollywood, and so much more that the Imperium never would have had the mind to imagine.
In addition, and likely most importantly, we have not been touched by Chaos at all given our isolation from the galactic war in general. Even beyond all the rich amenities we have that likely don't exist across much of the Imperium in general we are apparently in a "null warp bubble" and this could be the one place people could come to get away from it all.
In the end I would see us sending a tithe of both manpower and agri-resources and our planet slowly (or rapidly) becoming a paradise world. In many places around the world we would see paradise hot spots thrive given the infrastructure already at our beaches and resorts and grow even more. As you move away from those hot spots you would see agri spots thrive and grow even more rapidly as we begin to look to feeding a galaxy at large.