I had been vaccinated at the age of 2 or so. i never had chickenpox. will it make me immune from chickenpox for lifetime? else how could i prevent it?

Varicella booster: To reduce the incidence of breakthrough varicella and shingles, a second dose (booster) is recommended. This booster is usually given at age 4 or 12, but can be given at any time providing a minimum of 4 weeks has passed after the first dose.

Mostly yes: It should but no vaccine is 100% protective lifelong. Human immune systems vary and the response/memory wanes over time. You can get a blood test to check your immunity to Varicella. Also, you may still get Shingles later in life due to the waning immunity issue. Shingles is the same virus as Chicken pox.