I used to fast for around 2 weeks only till my high school..I was much unconcerned and immature (still immature).. Now I want to make up these fasts. How should I make up these fasts I missed which are not just a few and not countable?
In general I would recommend that you ask for fatwa directly from scholars or religious organizations/institutes. The person in question could then ask for more details if needed to give you a more accurate answer. I am not a scholar or qualified in any means to give fatwa, but I can only transfer what I heard.
As far as I heard (here is an example but it is in Arabic), fasting days that were intentionally missed out of carelessness are not to be compensated. Instead, you should repent and ask God for forgiveness. It is recommended that in that case you should try to fast a lot of Nawafel (e.g. Monday and Thursday, or the 13th, 14th and 15th of each lunar month, etc..), and do many good deeds. Surat Hud [verse 114] says: "And establish prayer at the two ends of the day and at the approach of the night. Indeed, good deeds do away with misdeeds. That is a reminder for those who remember."