Well this is the server problem, you should contact its admin or: 1. Make sure you spelled correctly 2. Make less/Few words 3. Don't use /register password CONFIRM password, use /register password password just write it twice without any additionals
thats all, and step 3 should be who helped, i think..
*Ahem* This is not the only site that learns you every thing you want! use http://www.google.com and type your question and you will get alot of answers...
Well, there are few ways: 1. Uninstall any mod or 2. Delete the .minecraft folder or 3. Make sure that your computer is safe without viruses or 4. If you have weak drivers update it (this helped me once) or 5. Make more free space (That also helped me once) or 6. Use the default texture pack (may help)
And i think thats all,,,, hope you solve your problem.