In addition to the wonderful things people have already said...
Firstly, spend time with Jesus daily more than any other person. Secondly, get baptised in the Holy Spirit and empowered from on high. Thirdly, trust and believe God's promises that as a believer your "old nature" was crucified on the cross and is presently dead and inactive and powerless to stop you - also if you haven't been water baptised get water baptised and understand the meaning of it (see Romans 6). And then take whatever practical measures you can to try and avoid any situations which would put you in a compromising position, in other words if drugs is a problem, stop hanging around druggies..it's not rocket science. And lastly, realise that Jesus loves you very very much and meditate upon that and His person until that truth is ingrained into your soul. Only God's love can really replace and satifisy whatever it is you are lacking in your life that causes you to turn to sin. That comes by developing strong fellowship with God, which comes by doing the first step I gave above.