drawing lewis structures.?

Your question boils down to this, "Which element is the central atom?" The element which is chosen for the central atom is usually the one with the lowest electronegativity. Of course there are some exceptions. Also, hydrogen is never a central atom. Even though hydrogen is occasionally a bridging atom, it occurs infrequently in simple molecules and so we say that H is never a central atom.

In the case of HNO3, even though H has the lowest electronegativity, it won't be the central atom, so the element with the next lowest will be. That will be nitrogen.

H-O-N=O

........|

.......O

Generally, you can use the following rules if the compound has an overall NEUTRAL charge:

H - makes one bond

C - makes four bonds

N - makes 3 bonds

0 - makes 2 bonds

F, Cl, Br - one bond

Two bonds can be one double bond or two single bonds. Four bonds can be two double bonds, four single bonds, or a triple bond and a single bond..note that the total bonds adds up to 4.

Carbon will usually be the central atom...N may also be and even O can be the central atom as in H20.

For HNO3, I am going to let N be the central atom:

O

II

H-0-N-O (minus charge on this O)

The N has a +1 charge, the O on your left has two lone pairs of electrons, the O on your right has 3 lone pairs of electrons and a formal charge of +1, the O on the top has two lone pairs of electrons....you can see a photo of this lewis structure on this page: http://books.google.com/books?id=ZCXPte7PL0EC&pg=R...

Typically the central atom is always the first element in the compound unless given the central atom or hydrogen cause it can only fit 1 in its octet. typically you count the electrons and determine a structure that you believe makes sense like can this be linear/bent/pyramidal/trigonal planar cause it has 3 or can it be a tetrahedral cause it has 4 just guess and chck really.