You can not directly start with CPA bidding strategy as it requires historical data. All campaigns start with CPC and once over 30-50 conversions received, within a month, then you can migrate to CPA.  It totally depends on your campaign goals. In my opinion, CPA bidding strategy is the better option. CPA uses Machine learning, AI and Historical data to determine when to show ads and how to increase campaign ROI.  If you have a limited budget or new to Google Ads, CPC might be the optimal choice.  CPC can help you learn about keywords that bring sales or conversions and keywords that need to avoid.  One of the advantages CPC provides, it gives total control to advertisers the amount wants to pay for an ad click.