A lot of time gets wasted during the day surfing the internet, talking on the phone, watching TV and/or playing video games. Spend a lot less time doing these things.
Step2
Spend more time reviewing class/lecture notes, reading text books. Set aside a structured, consistent block of time on daily basis to be used for studying and doing homework. If you finish homework before the block of study time is finished, then use that remaining time to do EXTRA reading and additional studying. Study ahead of class materials.
Step3
If your friends do not get good grades, make new friends who do get good grades and learn from their behavior and habits. Osmosis works.