It's no problem that you aren't used to english. This is an awesome place to learn!
I usually just get on without a mounting block from the ground by myself. Sometimes I will have someone hold the other stirrup for me for more security but that is not necessary. Of course, on some horses, it might be better, and if you are just learning how to mount english (there isn't as much to hold onto as western) it might be a good thing but it is simply personal preference. Make sure when you get on you are not gripping just the saddle, it shouldn't slip that much. It might slip a little bit (which I will get to in a minute) but then you should readjust it with your seat before you have your horse walk on once you get seated. Make sure you sit down slowly when you mount -- comfortably, as you would in a western saddle but just get used to the new feeling of the seat if you aren't used to it yet. When you mount, hold on to the horse's mane and the back of the saddle. It might help to hop a couple times to get more support.
Now for the girth...
The girth should not be too tight around your horse but it is crucial that it is not too loose. This is dangerous, as I'm sure you know... same thing as western. You should be able to fit 4 fingers into the side of the girth. But you should not be slipping from side to side and feel like you are about to fall off. If you are not sure have someone that is familiar with tacking up english to help you. If you use the same saddle and girth frequently, eventually you may be able to just memorize which holes are on each side and go from there (well, if you are riding the same horse in the same tack). For me, I think I'm on hole 5 on each side -- my girth is a little big on my horse!
I hope this helped you. Good luck and by the way, it's awesome you are working with several disciplines.
God bless, englishaqh (: