if anyone could give me some tips in what coaches look out for in a trial, general tips etc
1. Talent and technique. Coaches are picking a team to win, and at the next level up, those without correct technique get exposed and found out very quickly.
2. Communication and running between wickets.
3. For bowlers, the ratio of good balls / bad balls.
4. Picking a balanced team. You can't have a team full of patient openers, a team full of biffers, or a team full of leggies. A good team has a bit of everything, and can adapt.
5. Energy and skill that players have in the field. Coaches always want fielders who are keen, attacking, well skilled, with a positive team attitude.
6. Attentiveness. Coaches like being eyeballed by players when they are talking to them, and like it even more when a player listens and takes the advice they are given.
If you are 15 trying out in an U19 team, you would have to be extremely good to have a chance against others who most likely have 3 years more experience than you.
All other things being equal, if you have 2 kids available for 1 spot, most coaches pick the older kid first, because the younger kid 'still has next year'.
I'm not being negative, because if you have the full package, you will make the team regardless of age, but be realistic about your chances.
If you haven't made the team in the past, you should have an idea why, and know what you need to improve on. If you are unclear on that, politely ask the coach what you need to work on.
Just go into the trials with no expectations, relax, smile, do your best and enjoy the experience. Good luck!