The following information would help:
1. What version of El Capitan do you have?
2. What exact Mac model are you using?
3. What about your EMail program? Does it connect fine?
4. Have you tested another browser? Google Chrome is an "easy" one, and is fast.
5. How are you able to make the post above?
6. Do you make any backups to an external device? Time Machine is OK< but a backup with either SuperDuper! or Carbon Copy Cloner is better.
7. Don't think it makes much difference, but how much free space do you have on your internal drive? Also, have you done any disk cleanup/maintenance/repairs?
8. Do you have MacKeeper and/or CleanMyMac installed. If you do, get rid of them IMMEDIATELY, as they cause way more harm than good.