This is just normal stuff in today's tech world. My own server logs shows numerous attempts to gain access to my webserver everyday...it's not aimed at me in particular, it's just brute force hacker scripts. I would imagine that's what's going on with your site as well...I would be surprised if anyone is not getting this to some degree.
There are a few of simple things you, and anyone can do to help protect your blog.
1. Use a good, long, random password. That's the best defense.
2. Change your admin username to something other than admin. Hackers know that 90% of all sites and servers out there use either admin or root as the username, so that's what they target, then they only have to discover the password. Giving them your username is like giving away half your site security. So, change your admin username and double your security.
3. Keep your WP source code and all plugins/themes updated.
4. Only use plugins/themes that you trust.
There are lots of other things you can do, but these will keep your blog safe...the server is another story...hopefully your host is taking care of that part.