To start tonguing, practise the syllabul "taa" - you might have been taught this already but keep practising as your sound will improve.
Stamina comes with practise. Try mouthpiece buzzing. Hold the mouthpiece at the end of the shaft to stop pressure and buzz for 30 seconds at a time. Do this for 5 minutes at the beginning of every practise.
Your embouchure is what ever is comfortable to you. For range work try and play c maj scale but start from the top of the scale and go down then up again. Keep practising this until all the notes are correct. Then go up a semitone (Db) and play that scale and keep going. This will build up your range.
I hope this helps and I hope you're not one of my pupils!!!!!!!
Andy