Mixing the raw materials yourself is pretty risky because getting the perfect ratio of materials is going to be pretty hard to do, and that can affect the finished product quite a bit.
If this isn't a huge project, your best bet is to buy concrete mix in bags. Good old QuickCrete gravel mix is the best choice for this. It has the portland, sand and gravel already in there. Your other option is site-mixed or ready-mix trucks...Site mix trucks are pretty handy for small loads of a yard or two, and are fairly economical. You'll usually spend about the same on bagged mix that you will on truck mix if you get to yard+ quantities, and the truck mix is easier and more controlled because it takes a heck of a long time to hand mix over 30 bags to equal a yard.