SMD component clearance limitations and guidelines?
Are you planning on using PCB fab house and then hand-assembling??  If so, 0402 might be a tad ambitious if you've never done anything like this before.

Each board house will have a list of specifications of what they can make.  Some have two sets of rules or more -  rules that can be used for fast and cheap, for fast and expensive, and for anything you'd like, but it gets slow and fairly expensive.

http://www.pcb-specification.com/uk are the rules for pcb-pool.com--you'll see them if you click "Design Rules". They give you files to use to set up design rules in Eagle and in Altium, two popular schematic and board layout tools.  They'll even include a free metal stencil for solder paste.

If you're planning on using a fabricator, make them earn their NRE's!  Send them your design, mention your concerns, and ask them for advice.  It won't cost you a dime more to use their expertise in this way.  If you plan on a pick-and-place process, be sure to ask them about where they want their fiducial marks.

Lastly, don't be afraid to post the board on this site, and ask for a quick design review.  Be sensible, though, and make sure you provide good detail for the problem areas you suspect, and maybe tightly focus the question on that.