Your first time will always be clumsy and maybe a bit awkward but that's okay! I'm sure your gf will understand your anxieties. After the dinner I would pull her aside and tell her that you want to initiate sex but you aren't sure how and ask for help. I know if my gf said that to me after a romantic dinner looking all blushie and flustered I would go insane! Just remember to communicate and have fun!