I think you might be thinking about two different things here. Most concrete counter are full thickness "slabs" that are made off site and then installed like a granite counter. As a DIY project these are far beyond most people's skill set.
Replacing your tile however would be a great DIY project. In the last few years I have done a lot of very large tile installs. These give you the look of a granite slab but at tile prices.
I have did a ton of concrete countertops by myself. It's easier when you have help though, and a shop with all the right tools....... And several years of experience under your belt. Making your own concrete countertops may turn out to be a little more involved than what you think.
I just reread your question and you seriously need to know what you're doing before you start pouring concrete over your existing tile countertop, or I promise it's not going to turn out good. There's too much for me to type that you need to know before going that route.
It is complex but if you want to do it then do your homework and go for it. The quality of the forms used will have a large effect on the end result. Remember knowledge is power. Google the steps and read enough to form your plan then as I always say. PREP is the key to successful results! Good luck
Check it out on "Pinterest " because they have step by step on how to's.