The commonly accepted wisdom is to focus spec characters attributes based on  a single job, pvp, industry, mining, marketing etc and max out the relevent attributes for that job. 

If however you want to be some waht more flexable and enjoy the game spreading out your attributes will help, download perptuum planner and look at the attributes related to the extensions you will be mostly intersted in. 
If you have allready built your characters and spent any ep DO NOT delete the chars the ep will be lost.
Deciding what extensions you need I would suggest firstly looking at what modules you want to use and aim for the min reqs, or for production manu eff, refining eff etc. 

Support extensions cpu, reactor and accumilator and accumilator recharge time are allways worth while if you mean to use any robots.

Very Basic but realy any thing more comes down to what you want to do exactly and your play style.  Hold off spending EP untill you are absolutly sure of what it is you want to be doing.