Here are a few hints:
For everyone; Anchor your tongue on your side teeth. Ideally the teeth should be apart enough that the tongue can slip into the space between the upper and lower molars-- just behind the "pointy" canine teeth. This keeps the movement limited to the far front and mid-back of the tongue. It also keeps air from puffing the cheeks and avoids overly-tight face muscles. This helps your tone.
For clarinet and saxophone: There are two methods of tonguing-- "tip to tip" and "anchored tip." Tip to tip means the tip of your tongue touches the reed with a "d" or "t" syllable. Anchored tip means the tip of your tongue stays touching your lower teeth, below where your lip curls over them, and the reed contacts your tongue farther back from the tip, like you were saying the "ch" sound--- "chu". The way to tell which method to use is simple. Stand in front of a mirror and put your finger right in the middle of the "ball" of your chin. Then try to touch your finger with your tongue. If you can't touch your finger with your tongue, use tip to tip tonguing. If you can touch your finger, use anchored tip.
For flutes and brasses: the tip of your tongue touches the front side of the ridge that goes across the roof of your mouth just behind your front teeth. At no time should your tongue touch your upper teeth.
Double touguing for sixteenth notes and faster:
Note-- for this to work the air must move quicker than normal.
Flutes... the syllables are du and gah, so four sixteenth notes would be du ga du ga. Practice until you can say this smoothly and then try playing it on one pitch over and over.
Trumpets --- ta ka is the usual syllable combination for double tonguing on trumpet. Du gah can be use for a less percussive attack.
Trombones/Horns/Baritones/Tubas -- du gah is probably better for you.
Single reeds-- you can "fake" double tonguing using anchored tip and the syllables chu kah. Practice this without your horn until you can do it. It takes more air than normal to make it work.
Flutter tonging. The trick is to move the "side anchor point" up onto the upper teeth and move the touching point just behind the ridge in the roof of the mouth. Then put a lot of air through very fast. Try this without your horn until you can do it, then try it on the horn.