setup GA4 and Google campaign for my two websites

I have 2 sites....1 with 400 products and another with 40 products....the same products from first one. For the moment I need a person to help me to setup ga4 for both sites either google tag manager, for second site ....with 40 products to put on google merchant ...and 3 problem to learn from you how to setup a google campaign from 0 and adjust it when the results are not good.