How often you should practice? Tthat can vary depending on your goals and your level of fitness. I would recommend for a beginner yoga student to practice 3-4 times a week, for 1-1.5 hr. each time you get on your mat.
Before the class - avoid eating 3 hours before the class, come well hydrated so that you are ready to sweat, bring 2 regulars size towels, one for the class one if you plan to shower after the class, bring water and mat or rent it at the studio.
During the class - remind your self you are there for your self, remember to enoy your breath what ever it may be, look in your eyes and remember to smile and love what you see, keep an open heart and mind, it is cleansing ground for our emotions, try and set positive inntentions before and after the class.
After the class - take a few minutes after each class to check in with your self and see how you feel. Congratulate yourself for doing a great job in class and taking good care of your body. Acknowledge that you have more oxygen flowing through your blood, more room in your joints and that means more bounce in your step. Give loving kindness to everyone you meet.