In most cases, the more access to credit the better your score. BUT - you need to use only 30% of you credit line or less. For example - if you have a $1,000 credit limit - do no use more than $300. You don't have to use each card all of the time - just twice per year will keep them active (maybe lunch on your birthday and a tank of gasoline some other time during the year).
Another thing to keep in mind is that you need to be able to pay them off each month. PERIOD. If you can't pay it off at the end of the month - you don't need to purchase the items with your credit card. Also, by paying the entire balance each month you avoid paying interest with most credit cards.
I have paid cash for everything for so many years that my credit score didn't really exist. I started with a secured card from USAA. I use the card for all of my day to day purchases because it offers better protection against fraud than a debit card. At the end of the week, I pay off the charges. I now have 2 unsecured cards as well and in about 6 months have boosted my credit score by 85 points.
Good luck! You might also sign up for a free account at CreditKarma.com and you can follow your score and get other suggestions.