Hi mathiasfiedler,
Welcome aboard! You're not alone - these are pretty common questions folks ask when they get started.
The tiles print out with 1/4" sides. They're designed to wrap around the foam core, and will give you a consistent depth, even if your foamcore isn't 1/4" thick. In fact, most folks use 3/16" or 5mm foamcore.
The floor tiles come in dimensions that are multiples of 3" and either have a 1" grid, 1.5" grid or no grid on them. I personally always use 1" grids. I depends on the games you play. There are small 3"x3" floor tiles, hallway 3"x6" floor tiles and large 6"x6" floor tiles. They're provided for flexibility in layout. If you have the TLX Planner, it can be a great tool for figuring out what you need to make specific maps. If you don't have that yet, a piece of graph paper will serve you well - you can draw out your map using one square == 3", and that will give you a feel for how many of each tile you'll need. To estimate how many posts and walls you'd need, you can doodle that on top of the map too. The planner lets you do all of this in a very convenient fashion.
If you're just starting out, my best advice is to start small. Try making a 6" tile, four 2-way posts, and four 6" walls, and you'll be able to see how it all fits together. Some folks are able to try making tons of everything. It burns me out when I do that. It's also great when you've got a particular layout in mind, rather than just making stuff for the sake of making stuff.
The two biggest things that messed me up when I made my first set (also the lost halls set) was (1) my printer didn't actually print square, and (2) I didn't have a good system for making my tile edges square. I eventually bought a printer that printed square, after suffering through making three sets with slightly wonky tiles. I also have enough practice with making the tiles that the edges come out nice and square so they fit snugly together.
That said, even my early tiles still get used, and it's only my OCD that gets in the way of enjoying them as much as I should.
One last thing, if you don't already have some markers at the ready for edging, consider getting some. Black will work fine, and grey will probably look better. If you've got a variety of sets and you want to really make them look great, try to get a color that matches pretty well. Going with grey is good because it tends not to stand out so much as black, but it still covers up the harsh white edges.