Hello, I will tackle the items one by one below.
One thing that you need to keep in mind is that the recommendations may not relate to items on your website. Therefore, you should identify which items are from your website and which are not. There are some items that you can do nothing to rectify except from removing them from your website.
I am using WebPageTest.org and you can view the results here: http://www.webpagetest.org/result/130226_Q4_ES6/1/details/
There are a number of issues that I can see from the very start. First and most obscure, your website is causing a 301 (permanent redirect). This is a major issue and you should investigate what is causing it. If you are redirecting from http:// to http://www. I do not think you should be using a 301.
Loading of myblogtrainer.de and the redirection to http://www.myblogtrainer.de takes 1233ms, which is pretty massive. Even after the redirect, it then takes 3848ms to load, which again is absolutely massive. These loading times are one of the main reasons your website is taking over 6 seconds to load a 356kb page on a 5mbps connection.
This is before any major part of your website has been loaded.
The best way to solve this issue is to simply find another host who is better and has faster servers. Many hosts cram as many customers as they can onto a single server, causing it to be much slower. The server that you are on has more than 10'500 other websites on the same server. That is an awful lot.
Our system usually has one site per server/ip. Our smaller websites are added to a group server where we tend to have 10 or so sites per server/ip. This is one of the areas that google and other search engines will likely look at when deciding how relevant your website is. The more websites on the same server, the less relevant your website is deemed to be.
Leverage browser caching
You can view the details here: http://www.webpagetest.org/result/130226_Q4_ES6/1/performance_optimization/#cache_static_content
Browser caching is telling users browsers to keep certain items as they may be needed later. When the website is loaded again, all the cached items are loaded from the users computer, which makes it load a lot faster.
Using the link above, you can see the items that are not being cached in the browser. You can only effect the items that are on your website, not items on other websites.
Under Browser Caching in WTC, I select all options except the last 3 in General, All options except the last 2 in CSS & JS, all options under html & xml, all options except the last two in media & other files.
These are the settings that I use and they are also the same settings that I use on all of my own websites.
If you check the link above showing the items that are not cached by the browser, many of them are for gravicon, which I have stopped using as it seems to cause more issues than it solves. If they ever allow their images to be cached, I may add it to my sites again.
The only other item is google analytics and this can not be browser cached as it needs to load each time to log your users.
Changing your settings as detailed above, then clearing all your cache should solve this issue and allow browser caching of all the files on your website.
This is more of a nightmare. Redirects are when an address is loaded, but it forwards the user to another address. The more redirects, the longer the connection takes to complete.
To solve this issue, you should address the 301 issue mentioned at the top as that is one redirect as well as being a pretty major issue. Next, you should address gravicon as all of the other redirects are for gravicon.
The other issues that I would also address would be minify as that will reduce your css and js files down to one each, providing your theme and plugins work with it. If they do not, you may need to add the files one by one using manual mode.
I hope this helps you get started. Caching is like a whole new world. I have been using WTC on all my sites for over a year, but I am still learning new stuff. This is mainly because WTC comes with no manual at all and therefore you are left to wildly guess each new feature they add and how it works and should be used.