i agree with sage n btherl bout getting a singing coach. i am currently having lessons at FANG ZHONG HUA MUSIC SPECIALISTS music school so i can answer some of your questions.
firstly, singing DOES involve using the diaphragm as what my vocal coach says. nasal voices sound lyk britney spears' singing.dat is nasal voice,de voice dat u sing without vocal training.u wudn't wanna sound lyk her.if u sing wif ur diaphragm, de sound wud b powerful lyk an amplifier.mi tcher demonstrated to me how the diaphragm voice sounded lyk n i had this ringing in my ears for hrs after my lesson cos it was LOUD,VERY LOUD!!!!!!!!

(he was using de mic)
secondly,about getting a coach. if u can find someone who has gone to a music sch n has roughly picked up the techniques of his or her music tcher,u can ask them for help. if u r cashstrapped at the moment,trust me, u wont have enuf to pay for ur vocal classes. a grp class costs bout $188 but individual lessons are even more expensive, around $298-$338 depending if u wanna have the principal of the vocal sch teach u personally(which is beta cos de attention's all on u).i checked the website of de LWS(Lee Wei Song)Music Sch n i found out u had to pay $500+ for their grp courses.ur vocal class fees depends on how famous the sch or the tchers r.
lastly,singing high notes is easy if u noe de technique. try to think of de time u yelled at someone.was it VERY POWERFUL???was it VERY HIGH??? if u say yes to both,CONGRATULATIONS!!! y did i say dat??? cos singing high notes is like yelling, full of power and energy. if u noe howta yell, u bet u can sing high notes. its dat simple!!!!!! we use our yelling voice to sing high notes.don believe me??? TRY IT!!! u can sing leona lewis' bleedin' love in minutes!!!
P.S: improve ur pronounciation of ur words 1st before attempting to sing high notes. u have to "lock" ur jaws in de rite position to sing high notes like ur AH YEE WU EH ORH.jus msg mi if u gt unsolved singing qns!!!i'd love ta help!!!