tips regarding viagra

so I am planning to have sex in a few days with my gf. the very last time we tried was also our first time and I could not get hard due to anxiety I guess because I generally and normally get hard.

to avoid such situation this time I am planning to have a viagra before the sex.

can anyone help me with the accurate dosage. I'm 25M thin and short. I wanna know how long can 25mg of Viagra keep me hard. And how risky is taking it?