i need advice on girls

im 12 and have a new girl friend we make out alot and hold hands you know normal stuff but when we just talk i cant thing of anything to say i rly rly rly like and want to keep her but like she really hot and sexy and when we talk or she talks  and like we just sit there and its awkward and i feel like she pulling away also last week i got an erection and like i didnt exactly think about it but when she looked down when we were talking like shorts or something it was raised up and her face turned red and left anyway to hide erctions or anything like that and advice ion keeping a relationship attractive plz plz plz help i rly need it thx thomas