I have an entry level MTb with 21 speed gear ratio. I found out that i dont need that kind of complexity while riding. 12 speed seems light and compatible to my riding style. It has Shimano tourney right now so what can I do to convert it to a 12 speed ratio? Which derailer should I choose?
You'll need more than just a new derailleur. You will need a new rear hub (effectively, this probably means a new rear wheel), a new chain, a new cassette, and a new shifter. Possibly a new crankset.
Buying all this stuff as aftermarket upgrades will be unreasonably expensive compared to the cost of your bike. You'd be better off saving up for a new bike.
"shimano tourney" suggests your bike is definitely entry level, so dropping a lot of cash on it may not be a good use of your money.
As an alternative, if you simply want fewer gears, try riding around in one specific front chainring and don't shift the front. It is possible you might be perfectly OK to ride in the big chainring, or perhaps the middle one.
It is unlikely you'll choose the smallest/grannie chainring, it is too low for most riding.
If you can live in one chainring, then unbolt the other chainrings and store them. If your chainring is rivetted together, then this will be destructive and going backward is impossible. You remove the front shifter and wire, and it is possible to remove the front derailleur although you may choose to keep it as a chain guide, locked into position using the limit screws.
This will give you a 7 speed bike for minimal/zero cost other than your time. Not the 12 gears you asked about, but does achieve the complexity reduction/simplification.