I need a grid to help me model and design things.

Unless you can find a good modeler, then I need to know how to have a grid to help me design things like in tutorials. S.O.S!

If you mean creating models from scratch, good luck. The complexity of the tool goes along with the complexity of the model you can make. If all you want are spheres and cubes, there's probably a bunch of easy, perhaps even online tools you might find, but you just get junk. GIGO as they say. And no tool I've ever seen, even at GDC or SIGGRAPH, will make you a better modeller if you don't have the artistic skill in the first place (and I do NOT). If you can sculpt in clay, for example, there are computer analogs to that, expensive, but hey, you still need to be good at that.

Now, on the other hand, if what you're looking for is an in-scene grid to place and orient objects that you've imported, that can be done. Unity lacks a snap-to-grid (interactive) mode. But you can construct 'reference' objects and put them in the scene to help. Ex: the standard Plane object is a 10x10 grid, each element is 1 (meter) in size. So you can make a collection of those in various orientations and use them like 'rulers' or guides. I've found that essential for the work I do which is recreating real spaces. I made some that are in meters, feet, inches, etc. and use bitmaps to mark them up. Saved me a ton of time and headaches.

Have you tried Blender ?

Blender has hundreds of tutorials online :)

best modelling software

it's been asked many times on these forums, this is just one example and there are more.

take a look at the tutorials at 3dbuzz.com for all the different modelling software and see which seems easiest to you.

simplymaya.com and simplymax.com are two very good sites for the appropriate software and are an example of the kind of help you can expect for the software you choose.

the different kinds of 3d software exist to suit varying types of individual so it's a bit much to ask which is easiest and in fact to know what you've used before. most, if not all, have free trials and if you're a student could be free as in the case of alias software including maya.

3d modelling is not that easy whatever software you use except for maybe Poser, that can be easy but it's not really modelling if you're adjusting presets.

it's cheating and you would be like a big cheat and everything and i'm not even lying :)

Is there a free modeler that is easy to use and not as complicated as the previous ones I've used?