For an audition I have to play 2 octave scales and for E flat. I have to go to the low one(below the staff). I have an F attachment, so that helps I can play it in T3.5, but every time I play it doesn't sound right. So, how do I make sure it sounds like a note and not just air with rough noise?
As a general rule, I've always found the trigger Eb to be a little easier than regular F and E. But then again, I have a relatively large bore trombone. Is it possible there's something wrong with your F attachment? Does it sound perfectly fine on higher notes? It might be helpful to play higher notes and scales using entirely (or mostly) the trigger in order to get used to the longer tube and the different slide positions (only six exist with the trigger).
Can you play any pedal tones? The first 3 or 4 position pedal tones should generally be playable, and practicing them might help make the Eb not feel so low to you and give you a sense of the range of looseness your lips should have (that is, somewhere in between low E and pedal Bb).
I would also practice a lot using octaves. Go from F in regular first position (second line from the top in bass clef) down an octave to F in first trigger position (even though that note can be played in 6th regular position). Then do the same with E, then with Eb.
Always remember that you need somewhat looser lips and substantially warmer air for low notes. Think more "haaaaaahhhhh" rather than "fffffffff."
While there is a fair amount of divergence about the approach to expanding range, most pedagogues will stress the importance of connecting ranges together and minimizing motion of embouchure. A chipped note, or a note that doesn't speak, is caused by your embouchure not being in the right place.
One of the strategies I use to teach students having similar problems is taken from the Caruso method. The exercise is to start from a given note and move towards the side of the range you want without removing the mouthpiece from your lips. This is very important to limiting the amount of motion to connect the whole range of the instrument together. By starting away from the range you are working on, you develop the ability to go from one range to the other. This is one of the reasons why multiple octave scales are asked for in auditions, to prove that you have access to the full range of the instrument. And the degree to which you have this full range, whether or not this access is easy from one range to another.
Again, breathe through your nose and keep the mouthpiece on your lips for this exercise. No, you won't normally breathe through your nose, but one of the problems that can crop up is using the breath to reset your embouchure. I've almost never encountered a younger player who doesn't do this. This reset is what you are trying to avoid as it limits your ability to easily switch between ranges.
So in your particular context, you would start on the Eb two ledger lines above the bass clef. You will play sets of three half notes notes, but should not remove the mouthpiece between these sets, only breathing through your nose until all of the sets are done. It might seem pedantic of me to keep repeating this, but it is a very hard habit to break if you aren't used to it.
Eb -> D -> Eb | rest rest | D -> C -> D | rest rest | C -> Bb -> C | rest rest | Bb -> Ab -> Bb | rest rest | Ab -> G -> Ab | rest rest | G -> F -> G | rest rest | F -> Eb -> F | rest rest | Eb -> D -> Eb
... etc. etc. Continue down to the lower octave. You probably won't be able to get through even the first octave at first, let alone all the way down to the low Eb. However, after doing this for a few practice sessions, you'll notice playing throughout the range becomes a lot easier.
You should do this with other scales as well. When you're auditioning, the most extreme note should not also be the lowest or highest note you can play. Do this exercise with other major scales (all of them preferrably!) but work your way down to Db, C, and Bb ideally. (B is basically impossible on a tenor trombone because the slide needs to have about 9 positions to reach that note.)
Also work on playing this exercise the other direction. Start low and go high. Start in the middle of your range and go up, then go back to the middle and work your way down. Eventually you won't rely on the crutch that is resetting your embouchure when you breathe.
While doing this very exercise, you can also work on developing your rhythm. If you have a metronome, don't set it to the quarternote. (IE: Two beats per note). Set it to the whole note (IE: You play two notes for every time the metronome makes one click.), or for every two measures. This forces you to keep the pulse yourself and lets the metronome act as an enforcement mechanism instead of you relying on the metronome for the beat.
Check your instrument is working properly, then practice. Same as how to achieve most things :-)
Here's a strategy you may not have thought of. You can actually play the low Eb, D, D and Db WITHOUT the F trigger in 4th, 5th, 6th and 7th positions - i.e. a full position further out than you would expect. This is a quite distinct and different technique to the "long 2nd" for trigger-E, "long 3rd" for trigger-Eb etc. The notes are stuffy, and have to be "forced", but they're definitely there!
If you practice these "trick" notes, the trigger versions will become much easier! But also check your F valve is adjusted properly, and the F loop tubing is unobstructed.