I'll second Autumn's notes on finding online tutorials, like she said it depends a lot on what you want to use the model for.

Most of my experience comes from low-poly modelling for game engines, so thats the perspective I'm coming from, but there are a couple of things I've seen newer people miss over and over. Definitely stick to quads and tris, don't underestimate the importance of UVs or normals, take the time to learn about them and how to adjust them (watch out for inverted normals in Blender as backface culling is off by default) Avoid redundant geometry that doesnt add anything new to the model. Make sure you have good edge flow (e.g. if you add an edge loop to the middle of the model does it appear predictably?). And also save yourself time and effort where you can - start by modelling half the character and apply a mirror modifier to make it symmetrical.