R ttest on multiple levels of a factor

I'm trying to perform multiple t-test on my dataset in r and got totally confused from the capabilities of apply functions, aggregate and for loops. My data is as following: I have observations which are different products. for each product I have multiple numeric variables, which I'd like to compare. In addition, I have 13 different categories of products. AND, I have another factor variable which differentiate between new, used, and old products. So a sample of my data may look as the following:

prodID Category Cond No. of instances Sales Time since launch aaaaa Sports New 100 40000 30 bbbb Crafts New 0 0 20 ccccc Music Used 20 1000 10

My goal is to perform the following, I want to output separately, for each Category (Sports, Crafts, Music etc.) the results of a t-test. This t-test should compare means of each numeric var, with the comparison of "New" mean to "Used" mean (i'm not interested in "old" values at all). So at the end I want to see the comparison of "Time since launch"m "Sales" and "Num Instances" betwwen new and old in Sports, then the same in crafts, the same in music etc....

I've tried it in so many ways, but in each of them (aggreagte, tapply, for loop) I had a different problem... It seems that I'm missing here something (I'm kinf of new in R. I used to do it in spss and used split file...)

Any help would be very much appreciated!!!!