I was in a similar boat when I started, I did my beginners course with a
recurve and as soon as it was over I went to
longbow, as its why I got into archery. As others have said above most
longbow archers are self taught with only the lucky ones being taught by someone who know how to shoot longbow well.
When I first started I was encouraged by other longbowers to try different sighting methods to see which one I was more comfortable with. Point of aim I never really got one with esp indoors as I found it odd not looking at what I was ment to be shooting at. The rubber band I found more appealing but since was only a rough sight that changed due to weather, temperature etc I didn't want the hassle of the bookwork. Though it did improve my shooting initially I found after a while I wasn't even noticing it so I removed it from my bow and shot instinctively and not looked back.
If you choose instinctive there are a few truths:
1) You will need ALOT of practice just to get the feel for where you are ment to be shooting.
2) Indoors you can be as good as any other longbower with sighting aids.
3) Outdoors you are at a distinct disadvantage compared to theose with sight aids esp when it comes to the longer distances, as you have no constant referance and a slight change in angle can make a big difference. So you'll need more practice.
I would honestly say that the longbow is the most unforgiving bow, but then that just makes it all the more rewarding. I would suggest alot of practice, talk to longbowers at clubs/competitions. We are usually a very friendly and chatty bunch and since most are self taught we don't mind giving tips to what we've found successful or not.
Lots of archers don't expect longbowers to be accurate in fact one of the longbow archers at my club sum up the three main disciplines:
Compounds get upset if they stray out of the gold,
Recurves get upset when they stray out of the red,
Longbows smile when they hit the target, as each hit is a small victory.
When I first started I found this to be a big help towards attitude as I could get a bit down that I was missing the target (which doesn't improve the shooting), true now I get annoyed if I miss at short range, but I still have the attitude at the longer ranges. As long as you don't get disheartened and enjoy your shooting then you will improve.
As for shooting tips:
1)Consistancy is key, find your prefered anchor point and stick to it.
2)Gently loose the string don't pluck it.
3)Hold the bow lightly in your hand, have the ball of your thumb against the handle, with your knuckles at an angle, just remember to hold the bow.
4)Take a deep breath BEFORE drawing, and as you draw breath out half slowly.
5)DON'T hold the bow at full draw for too long, not only will this decrease the strength of the bow it'll increase the chance of it breaking. Hold for max 3 seconds.
6)Don't adjust your shot for every arrow, find the group then move.
7)Whether you make your own arrows or buy them you will need the right spine for your arrow depending on bow weight and arrow length, and try to get them all within a similar weight.
8) When shooting at longer range bend at the waist, don't raise your arm. I see this alot at some competitions and they struggle to hit the target for as they raise their arm the draw length shortens, once pointed out they are much happier.
9)If you go to competitions or local clubs, other archers will gladly watch you shoot and offer advice. I went to a shoot once with other members of my club, and one was struggling, so at lunch time an archer on the same target pulled out spare arrows and tested her with them, finding the arrows she was using were over spinned by quite a bit. For the second half she used the borrowed ones and really improved.