An easy way particularly if you are not too experienced with plants is to buy packaged soil or a readymade loose mixture from your garden center or florist. In some neighborhoods, you can get bags of convenient size, and even specific mixtures prepared for various types of plants. (However, commercial combinations bought in hardware stores have not been invariably satisfactory; often they are very light, and those available are not sufficiently labeled for various plants.)
If your work or storage area is limited or your time you may find it convenient to buy quite small amounts according to your needs. From my own experience this is most satisfactory and, incidentally, I almost never have to add anything to the basic mixture I select at our fine garden center. Furthermore, most packaged soils or those from florists are already sterilized, and this is a real advantage.
But you may prefer to be more scientific about soil in order to bring to perfection some one group of plants. If so, get in touch with your county agricultural agent or state college horticultural department. From either you can get the latest information on soil preparation for the plants of your particular interest. You can also buy a soil-testing kit to get the pH reading of the soil you have, and then improve it according to the advice given. In some states, at no cost or for a small fee, your county agent will test your soil for you.