Need Very Serious help Experts please help

[zoccy]

I own a forum which got 10000 posts / topics and 3000 members

i placed tracking scripts in footer which gives me idea of how many vistors are on forum per hour and at any given time

according to which i have 10-14 members online per hour

my server load show always less than 1 near to 0.30 to 0.76

now yesterday i got an email stating that he will hack my forum and do some damage , for dont know why .

and suddenly after that email my server load was showing 1.80

and

PHP Memory Limit 128.00 MB
which was much lower

now i dont know how to know if someone is hacking or not ?
i dont know how to protect this ?
i dont know how to make this server load lower again ?
i dont know how to get imdediate email or notification if their is any hacking or sql database hacking attemps ?

can anyone help me with this please ?

i am on shared hosting server.

please reply