Transferring the money or keeping it in US does has no effect on taxes. Your residency status has. Assuming you are Resident Alien in US for tax purpose and have paid the taxes to IRS and you are "Non-Resident" Indian for tax purposes in India as you are more than 182 outside India.
How would it effect my Tax in US and India
If you are "Non-Resident" in India for tax purposes, there is no tax liability of this in India.
I have transferred an amount of approx 15-20k$ to Indian Account (not NRE)
By RBI regulation, if you are "Non-Resident" then you should get your savings account converted to "NRO". You may not may not choose to open an NRE account. To keep the paper work clear it helps that you open an NRE account in India.
Any investment needed ? Where do i need to declare if any ?
These are not relevant.
Note any income generated in India, i.e. interest in Savings account / FDs / Rent etc; taxes need to be paid in India and declared in US and taxes paid in US as well. There is some relief under DTAA. There are quite a few question on this site that will help you clarify what needs to be done.