How to start doing push ups?

hey guys, im an overweight guy i have a very good friend he is a gym instructor he made me a schedule which i follow tightly from monday to saturday and diet includes high dose of protein in the form of soya and whey and meat egg whites the problem is due to my body weight and low muscle strenth i can not do pushups which i have to do on monday i skip them i cant even go to my friend for help as a good instructor he is very busy person can anybody help me so anybody can help me in guiding me how to start pushups evrytime i try i fall down and i feel so embred! I lost about 6-8 kgs within just 20 days by petty exercise of 45 min daily can anybody help me please