Commonly you can only guess who did it. Google for "DDOS protected hosting" and transfer domain there
You will need to contact the server administrator to get the logs. The quickest way to tackle DDOS attack is to null route the server IPs.
There isn't too much that you can do if your host does not protect you from said attacks. What I would recommend is utilizing a DDoS Protected Hosting Provider. These days it's damn near a requirement to have DDoS Protection on any medium to high traffic website. A lot of kids out there attacking websites for the 'lulz'.
Check the logs where the traffic coming from, you will probably see an ip that flooding the website, you can null ip, change your website ip untill it stop.
You can know who did this attack if you are a server administrator, you can do that by take a look of income connections to your ports you should get a high connections from one ip or a same range of ip's, you can get protected by using provider who protect from this attacks, firewall, blocking ip's range (very simple way because he will get another ip's and you will attacked again), cloudflare, there is alot of methods you should contact your hosting provider.