The reason your mother or grandmother told you to be careful who you spend time with, is because whether it is on purpose or not, the people you are with become like role models. If you are around less than stellar people, then you will probably become a new AND worse human being than you were before.
So why not choose who you are with to become a better person? Why not choose to be with people that will make you a new AND better person than you were before you met them?
Consider what Paul writes in his 2nd letter to the Corinthians,
“Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new.”2 Corinthians 5: 17
Imagine that you make Christ your role model. Imagine you take step 1 which is to be in Christ. You become in Christ by accepting Him into your life saying it out loud and then being baptized. Then spending time with Him in prayer and fasting and praise is being with Him. As you do the things that Christ asks you to do such as the commandments and other instructions, you will become a new creature because you are doing new things. As you reach new heights in your faith and as a person living in Christ, your old person and habits will become a thing of the past because you will no longer do them – they will pass away. What remains in front of you is everything new.
Your first step then, if you aren’t already living a life in Christ, is to accept Him into your life. Once you have done that, carefully look at the role models in your life and how they are you making you a new and better person. If they aren’t making you better, you need to limit the time you spend with them or cut them off completely. Its tough. But the rewards are immeasurable.