i have a boost mobile phone i got from a friend and i have a tmobile sim card but everytime i put the tmobile card in the phone it says enter special code ( a subsidy lock)

Don't try to enter codes at random, after the 5th try the phone will be permanently locked.
All phones with a SIM card in the US are locked to their carrier. To get a subsidy unlock code you have to call Boost customer service. This essentially unlocks your phone and can be used with any SIM card, be it Boost, T-Mobile, AT&T, etc. even in Europe with carriers such as Orange, Vodafone, etc.

no , tmobile is a GSM but boost is a old system of CDMA anulog, they are totaly diferent,

NO! dont try it! i have tmobile and i tryed that and my phone got locked! With tmobile plans (sims cards) You can only use Tmobile phones! Thats the thing that sucks about them.... But you can find cheap good cells on ebay! Just type in tmobile cell phones, they have a wide variety! The reason it says you need to enter a code is because the phone is locked, aparently you tryed it too many times and it got locked. Try going to a boost mobile shop or tmobile, theyll unlock it for you for like 2-3 bucks

Good luck!
Kendra