Germany: Purchase a transport and place it in the Danish Sea and purchase artillery and place it on Germany (You could also put the artillery in Finland or Poland, but I prefer to put it here because your opponent may think you are preparing an invasion of UK and that may get them to spend that $12 purchasing Navy or Infantry for UK, which I’ve seen happen). Then you can use the 2 transports you have in the Danish Sea to pick up 2 infantry and 2 artillery to Leningrad, where Russia only starts with 2 infantry. You can use the fighter from Poland or the bomber from Germany for extra firepower, although there is a possibility of them getting shot down by the AA gun. If the Russians stack up infantry on Leningrad with their $12, you can use this same force to attack Baltic States along with forces from Poland. This allows you to shift some of your forces in Poland to attack East Poland and gives you more strength at the front. These transports will also be invaluable in the next round and beyond because you will be able to move infantry and artillery to the front much more quickly than their normal movement would allow. This is key in taking down Russia.
Allies: Give the $12 to Russia and purchase 3 artillery and place them in Belorussia. Germany will most likely attack Baltic States or East Poland and this will allow you to have a decent force to counterattack either of these areas. However, if the Germans put a transport in the Danish Sea with their $12, put the 3 artillery in Leningrad. The reason to use artillery here is that if they decide not to attack Leningrad and attack Baltic States, you have some offense to counterattack with while only having 1 less unit with a defense of 2 to defend with if they do attack. The key for the Russians is to have enough offense in their counterattacks to take out German armor. If Germany puts 3 artillery in Finland, purchase 4 infantry with Russia and put them in Karelia. This will force Germany to bring in the fighter from Norway or Poland or the bomber from Germany to put the odds in their favor. The Russians also have 3 infantry and 1 armor in Archangel, 4 infantry and 1 artillery in Russia & 1 armor, 1 fighter and 1 bomber in Moscow all in position to immediately counterattack if the Germans manage to take Karelia.
The most important battles, especially in the first few rounds of the game, take place between Germany & Russia. So no matter which side you are on you should use your $12 at this front.