I am looking for offline or online English-to-German dictionaries that provide example sentences for each entry. This would help me learn the usage of each word. For example: I searched for the word admit and it shows different German words, but without the usage of those words, I can not understand the proper meaning of the very English word in German.
I recommend you to use at least two web pages: one for translation and one for example sentences.
... you name it
Furthermore it's possible to search for words with the aid of google. Just type site:bla.de word.
For instance, use news sites such as focus.de or faz.net.
In addition to this you could use a monolingual dictionary for clarification:
I'm afraid but there is no good online dictionary for this purpose. Especially no bilingual dictionaries.
The already suggested dictionaries dict.cc and dict.leo.org are very popular, but in my opinion they are only good for people who speak both languages good enough and only want to quickly look up a word.
A better dictionary is Pons, because they at least categorize the translations according to different meanings of a word. (Choose "Deutsch<->Englisch")
Another dictionary that provides a few examples is Collins
Leo and dict.cc are good german/english dictionaries. Also, you can get the book Mastering German Vocabulary as a free pdf if you google it. It is good for clarifying the use of many similar words. There is also another book by a similar name which cannot be found online for free (as far as I can see) but I've bought it and it is quite good.
Not exactly what you are looking for, but linguee.de has real-world translations with context in both languages.
Though no D-E dictionary of which I am aware provides illustrative sentences for every entry (this is quite unnecessary and would be extremely costly) the Muret-Sanders Encyclopedic dictionaries have abundant illustrative material. I am a professional German-English translator and use the Muret-Sanders quite frequently. If you are looking only for German, the Grimm Wörterbuch is enormous, and a good resource. A dictionary such as you describe would be gigantic, running to hundreds of thousands of pages.
Another good resource is Farrell's Dictionary of German Synonyms (3rd ed, 1977, Cambridge U Press). It explains the usage of various German terms that translate the same English word.
DWDS ist ein großes, aufwendiges Lexikon mit Definitionen, etymologischem Teil und einem Teil mit Beispielmaterial (sehr viel).
Jeden der Teile kann man scrollen.
Auf der Startseite (dwds) findet man eine Kurzinformation zum DWDS (Digitales Wörterbuch der deutschen Sprache).
Disclaimer: I am the developer of the website.
A few days ago we added this functionality for the http://dreiartikel.de/dictionary
About 48K German definitions and about 51,5K English definitions. In both cases we are talking about words and short expressions.
The examples are coming from the European Commission's DGT Translation Memory project.
The big advantage of this is that the translation quality is at professional level and the examples help you build a very "hoch Deutsch" or technical/specialized English vocabulary.
One disadvantage is that more or less colloquial words like "Lust" don't find themselves in these documents that the Commission creates. Another one is that the examples are not always ideal because maybe they represent just titles.