hy i want to create a dc-dc converter circuit that will be able to convert 30v/2a into 12v/5a dont seem to have a clue on what to do tried using lm317 with transistors in parallel im only able to adjust the voltage not the current,
This means "no loss" = 100% efficiency. This is not possible.
Please give an efficiency value that is acceptable by you. Useful values are 80%, 85%... even higher becomes difficult and needs extra effort.
***
Generally this calls for a "switch mode voltage converter".
Becasue input voltage is higher than output voltage: look for "buck" or "step down" converters.
Theoretically a buck converter can do this. This simple simulation is based on using ideal components.
The LC input filter draws smooth current from the supply. Internally the converter has waveforms peaking over 8A. The capacitors must carry several Amperes going back and forth at several kHz. For the switch you'll use a transistor or mosfet.
There are many DC-DC converter modules on eBay at a couple of $US. Just try to double the power rating as the Chinese manufacturers have a habit of over-rating their products.