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I don't know if I'm tonguing right, as in single tonguing, in the right position. I get my tongue at the back of my teeth, and I thought it was the top of the gum-line. This really annoys me and I need to know how it should be done.

Here's an artist's rendering of my situation:

Sounds like you are trying to control the tongue by muscle, a no win situation. Here's what Mr Jacobs says:
http://www.windsongpress.com/almost%20live/tongue.htm

You will find you place your toungue in a different spot for different reasons:

1. The range of the selection
2. Volume
3. Shape of the note
4. etc etc etc

Note that if you are going for speed, that the shorter distance your toungue moves, the faster your articulation will be.

The important thing is not HOW you do it, but how it SOUNDS. I saw some x-ray movies of different brass players, showing what goes on in their mouths while they play. They were all different from each other, but they all were great players.
Record yourself playing various passages in various styles, preferably in a room where you will be performing. Listen carefully to the recording and think about what you would like to change. Then remember what you thought about the next time you pick up the horn. Don't even try to put it into words.
It also does no harm to listen to other players for ideas. (Listen to their playing, not what they say.)

Check out this article on articulation by Roger Bobo at our neighbors, TubaNews:

http://www.tubanews.com/articles/contentid-254.html

In my personal experience the most important factor is the use of air. If you get your support right, and more imortantly the coordination between tongue and airstream, you can get a clean attack using almost any consonant in the vocabulary. (For low notes I tend to use a sort of Loo articulation. Doesn't sound like one though..)
Get your air/tongue coordination wrong, and you can say T with the most perfect tongue placement possible and still sound "Splioo"

Translated from Portuges :

Play with Emotion