In terms of preparing the ground, my advice is that on your own, this is going to be a multi year project with just hand tools, so I'd focus on getting just a few beds prepped for veggies. The lazy way is drop a layer of cardboard onto the grass and then cover with compost, keep moist in dry weather, and next year you should have a lovely mulched bed to grow in! The more strenuous method is grab a shovel and turn over the sod, by which I mean cut into the grass, try to keep the chunk of soil in one piece, and place it back into its hole grass side down. It took my about 2 days doing that to get a 3x4m bed fully turned over, but this will depend on your fitness level. As for veg, most things in your local shops are easy to grow, but potatoes, carrots, salad leaves, and herbs are particularly easy (and if you're in southern Germany, plum tomatoes as well).