I want to convert my whole cash in my bank account to bitcoin but dont know if is a good idea for me or not.
You can not convert your Naira to Bitcoin directly with your bank account. You must use your Naira to buy Bitcoin, it's not convert.
If it is your first time to buy Bitcoin, let's try with small Naira as your first practice. If it goes well, you don't make any mistake, you can proceed a next one with bigger value.
Buying Bitcoin then storing it in non custodial wallet like Electrum wallet that is free and does not need big storage space in your computer. It is available for mobile devices too.
https://electrum.org/#download[GUIDE] How to Safely Download and Verify Electrum