How to implement getrawmempool? I’m trying to get all transaction ids in the memory pool as json array. I just finish downloading bitcoin core and do now know where to begin. Any help would be appreciated!!
You can use the command bitcoin-cli getrawmempool to get a JSON array of all transactions that are stored locally in your mempool.

Since v0.9 release you can also pass a verbose boolean to get more info related to the transactions in your mempool like size, fees, or ancestor/dependent transaction within the mempool. 

Do note that there is no one global copy of the mempool. The mempool is node dependent and stores valid transactions that your node has seen but has not yet been confirmed in the block. Since you have just downloaded and spun up your Bitcoin Core node, your mempool information might be limited when you compare it to nodes that have been running for days.