HELP! Need help with rental car & train tickets for early May

Hi! I am traveling to Spain in early May with my husband and another couple. We fly from Barcelona into Granada and plan on renting a car and driving through some of Southern Spain, staying in Granada, Marbella & Seville. Then we are planning on returning the rental car in Seville and taking the high speed train to Madrid. We fly out of Madrid to head back to the U.S. I hear mixed things about whether you need to buy train tickets in advance. Last time I looked, the train schedule still wasn't available for our travel dates, and we aren't sure what the cost difference is or the best website to use to book. What is the best way to book train tickets & how far in advance should we book? And what is the best/least expensive way to go about renting a car? Just thought you experts travelers might have some good insight. Thanks in advance!