Some people do it just to upset other people. Unfortunately such people exist in real life.
There are three ways you can deal with this as a new player.
1) Get out of Open. Join a private group such as Mobius/Mobius PvE or go solo, until you upgrade your ship a bit.
2) Just accept it, learn how to evade interdictions and combat. But in a starter sidewinder, you might have trouble regardless.
3) You died, you are back in the station. You want revenge, but no way can you do it in your little ship with your current level of skill. But there are players out there who can and will help you. Many will help you simply because they also hate seal clubbers. Check the station for other CMDRs, or send a local chat, letting them know there is a seal clubber in system, and would appreciate some help, either an escort or someone to go collect a bounty. If nobody around or nobody willing to help, get on the social networks, either here, or reddit, or elsewhere, and send out a call for help. Be careful though, most places have zero tollerance for "naming and shaming", so the way to deal with it is to let people know that you are in trouble, and need help, where you are, and if you know, the type of ship that attacked you. Someone might be able to respond and help. You can give the CMDRs name directly to whoever comes to your assistance (if you managed to get it).