First things first, you should build your Supply Yard as soon as you get 50 fuel, and your Motor Pool as soon as you get the next 45 fuel. Chances are, you're either seeing infantry Half tracks or Armored cars on the field. You fight them both the same way - either run to the nearest cover/building, or run back to your base.
Now, as your Motor Pool finishes building, look at how much fuel you have. If you're anywhere near 30 fuel, just wait for it and then build an M8. If you've been pushed off the map completely, which isn't uncommon, and you don't have enough fuel for an M8, make an AT gun (you should be able to afford an M8 shortly thereafter). When your M8 comes out don't forget to upgrade armor skirts and the gunner on top, in that order. You have to immediately watch out for a few things.
Teller Mines.
One teller mine can kill an M8. If you're not sure of your opponents doctrine, or you know he's tank hunters, you have to watch out for these. Giving one of your Engineer squads a minesweeper is definitely a great idea, and not running your M8 into those 'obvious hot spots' is a wise choice as well.
AT-HTs
These things are the biggest fly in the soup you can face. If you charge your M8 in unsupported, this can easily immobilize your M8 and kill it, by itself, which is why you have to watch the hell out for these things. If you see one, immediately reverse your M8 away from it and get back to your support units. Once you get AT gun support, you'll be able to use your M8s a little more liberally, as AT guns can easily chase away AT-HTs and give your engineers a window to repair them.
The Marder
If he has a Marder up by the time your M8 pops out, he either straight teched to it, or you sure lost the fuel war. One Marder, by itself, is very easy to counter. Use your M8 to circle it, constantly staying out of its very narrow firing arc, as seen here;
As your M8 hits the field, your whole game plan shifts. You should still be trying to outcap your opponent, but you may very well have the ability to force your opponent off the map. If he is unable to immediately counter your M8, move to his remaining fuel points and decap them, using the M8 to chase away infantry, infantry Half tracks, or Armored cars. Your engineers no longer become cappers, but emergency repair crews. Keep your engineers about a screen away from your M8, so if it becomes immobilized by an AT-HT, you can send them in to try to repair the engine damage before you lose the M8.
However, if your opponent has the appropriate counters on the field, you need to eliminate the threat first, and then go after his fuel. Marders can be circled, but AT-HTs need to be taken out by luring them into the firing arc of your AT gun. Now for some good news. His infantry is basically useless now. Unless he's using the AT-HT + AT grenade combo, he has no infantry that can reliably damage your M8. If you keep your M8 at a distance, supported by an AT gun, schrecks simply aren't accurate enough. Schrecks in infantry Half tracks need to be shot down by AT guns. For the most part, your Riflemen no longer engage the enemy, just staying behind to cap and maybe chase off a Kettenkrad.
The mid game build order
M8 - AT gun - M8 - AT gun - M8 While this build order isn't set in stone, it provides you with great mobility and even better anti-tank power. Your M8s are the lifeblood of your army. They're able to hop from skirmish to skirmish, and are very effective against not only light vehicles, but infantry as well. The only PE non-doctrinal units they cannot fight are panthers and Panzer 4s, and they should not engage AT-HTs unsupported. Your M8s choose when and where to engage the enemy, and although a Marder can knock them silly if one gets stranded, they're able to out maneuver most PE units and get to safety if things go sour.
Once you get a team of 2 M8s (and an AT gun) up, you can use them a little bit more freely, taking a few more chances, due to AT gun support. Don't do anything stupid, but generally you should be aiming to take out unsupported Marders by circling them, or using an M8 + AT gun combo to knock them out. In this situation, you should use your riflemen as protectors of your AT gun. Losing a 57mm AT gun to a few Panzer Grenadiers can really hurt you, so don't let those greedy krauts get their hands on your anti-tank weaponry.
Now, hopefully, you have the upper hand, and you can begin taking their fuel. Marders have a fairly steep fuel cost, and you want to deny them the ability to make panthers or pz4s, so keep harassing their fuel, and keeping your army mobile. If you can catch a pack of Marders 'on the move,' get in there with your M8s and start circling them.
As for resource management, you want every M8 to have armor skirts, and as many .50 caliber gunners on top as you can afford. If you have the choice of armor skirts or a gunner, always always always go with the armor skirts. You still shouldn't be spending munitions on things like grenades or flamethrowers, so you should have enough munitions to get armor skirts, at the very minimum. As you gain more and more map control, you should have a much better munitions income.
