You open 1[diamond] and Responder bids 1[spade], natural, showing 4+ spades and 6+ HCPs. What next?

Generally your first instinct is to rebid you’re your diamonds which is not a good idea.  On these hands you want to start bidding out the shape of your hand at the lowest possible level.  Start with a 2[club] rebid, showing at least nine cards in the minors and 12-17 HCPs.  Then plan on bidding diamonds again at your third bid.  By introducing a new suit and then bidding your original suit at the three-level you are indicating that you have a hand that is strong enough to want to investigate a game contract, even though you do not have a fit with partner.  Bidding diamonds, then clubs and then diamonds again indicates a 6/4 hand with 15-17 HCPs. 

These auctions are difficult for new players in the beginning because they are not just some rote auctions that you can memorize.  They are about using the order in which you bid your suits to differentiate stronger and weaker hands.  Learning to convey that information is a bit of an art…but you’ll learn the techniques.