Its more like advice

I've been invited by my friend to take pictures of him showing his pictures for the first time in a gallery he draws and paints a lot. So he is going to have a small opening ceremony and he invited me to take pictures. Being really new into this I am a bit afraid but I relish the challenge into this. Any tips how to do it? And what to use 18-55mm or the 55-250mm lens ?

bring a flash

If it is a small gallery and you're using a crop body, that 55-250 might be too long even at the short end.
Always, take the glass you have. read: Take Both

Capped, that 18-55 will make it through the entire night in a pocket.

Yeah, I agree with the advice to take both. Between the two, I'd think the 18-55 would be more useful for what you are doing unless it is a big gallery.

Thank you for the advice guys :

P.S. What does Crop body mean is it bad ? IS the 60D bad model I mean for cropped pictures or ?

Crop refers to the Sensor in the camera, not the picture. Crop body cameras like the T3i, T4i, 40D, 50D, 60D 7D have a smaller Sensor than the Full Frame Body cameras like the 5D or 1D. So, they end up having a 1.6 magnification factor to the Lens Focal Length.

Your 55-250mm on a Crop, gives same view as a 88-400mm would on a Full Frame. Good for reaching out further, usually outdoors.

Your 18-55mm on a Crop, gives same view as a 29-88mm on a Full Frame. Good for up close indoors distances.

Full Frame cameras have sensors that are same size as 35mm Film was. That is why they are called "Full" Frame. Nothing bad either way, just different!