Best bet for a beginer is not to try and mix your own soil, but just buy tried and true mixes. Fox Farm Ocean Forest is a popular mix although there are many others. Cheap mixes tend to have bugs, or be 'Hot' ( excess Nitrogen ) or just be nothing but composted sawdust,.
Do NOT get soi that has time release nutriants such as miracle grow, as they tend to burn and are too rich to use effectively.

Good ones I have tried are :
Fox Farm Ocean Forest
Botanicare makes a good mix.



Good drainage, presence of Worm Catings, Kelp meal, Blood and Bone meal, trace minerals, perlite etc are all needed or good indications of a premium mix. Talk to your freindly neighborhood hydro store person and I'm sure they can streer you right. If they have a bag open you can generally judge by appearance and heft. Squeeze a handful and see if it compacts or bounces back after being squeezed, You want a fairly light mix, as a mix that is too dense will not allow for good root production and efficiancy and restrict drainage.. A core or pete moss based mix ( core being preferable) rather than one that is just wood chips composted with other things.

Hope this helps