To start your first visit to the TLG , you should click on "Establish a New User Profile". This will enable you to save your searches, and to set up your preferences. Also, on the first page, you will see a choice between unicode and beta. Most computers have a "unicode-capable/OS": so you should enter through that door. This means that you don't have to install special fonts on your computer and that you can copy and paste the Greek into another program without losing the accents and formatting.
To see a listing of the authors available, click "Search the Canon by Author" -- on the left side of your screen--and, once you're there, click on "Complete List."
To access a text, type in the author you want, and then follow the prompts. Notice, that you can tell the TLG in the settings for your account that you want to have a connection to Perseus, and this will enable you to access a translation. It will also allow you to access dictionaries -- just click on the word you're interested in and Perseus will provide you with the dictionary form of the word. On the left side of your screen, you'll see the question: "Perseus links?"
To search the Greek texts for particular words, click on the "search selections" button. Once you're on the new page, tell TLG what you want to search for. You can do a simple or an advanced search.
For further guidance, click on Help in the TLG.