What is the best method of upsizing an image? I wish to crop the image as well.

What is the limit of upsizing in percentages - one method I found was in increments of 10% and bicubic smoother.

It is for printing purposes for a client.

I wish to print online, so therefore i have to prepare for print - it is quite daunting.

Appreciate any comments or pointers, particularly any tutorials that I could have access to.