How long is your program? A 2 year or a 4-6 year type program?

The first semester can be tough with the course work, and graduate programs tend to be an adjustment in your method of thinking - if you've been able to get by with just memorizing stuff previously, grad school can be a bit of a shock.

I agree with everyone's suggestions of consulting other professors and the rest of that... and I think what got me through grad school was having a professor that I worked really well with & a project that I truly enjoyed, was passionate about, and LOVED. I'm currently doing a post-doc that's meh... and I hate it and pretty much want to do any other career than this now. But I just need to find a lab that's a better fit, or get a better attitude.

But if you're truly not sure that this is what you want to do... Leaving after the first semester is really not a terrible thing. You tried it, it wasn't for you, and you didn't waste any more time or resources on it. I knew people that 4th year into grad school decided that they'd made a terrible mistake and this wasn't what they wanted to do with their life at all... They pretty much restructured and went into science policy or lobbying - but it really wasn't something they needed a phD for.