I'm no expert since I too am striving for my first publication, but I have been building and submitting puzzles for a few years now, so I have more experience in puzzle-making than when I first started.
My first recommendation would be to read over the links in Sage Advice
, then read them over again. Read over the Basic Rules
too. The rules can be bent, but I'd suggest making sure you've got a good handle of them first before getting too ambitious.
Then, get yourself well-acquainted with the Cruciverb-L e-mail list, where you can e-mail other constructors. Perhaps some who are real veterans in the field would agree to mentor you on whatever puzzles you create.
Next, if you don't have it, purchase the full version of Crossword Compiler. It's a must for pretty much everyone in the crossword construction business. You should also look into registering for both Xwordinfo and Cruciverb Gold memberships. Those will give you access to tons of past puzzles where you can get ideas for puzzle construction. They're not free, but they are worth it. Not only will they help you with filling the grid, but more importantly, they might also help steer you away from themes which have been done in the recent past. Editors are less likely to accept your submission if you use a theme that's already been done, or if you build a themeless grid with "anchor" entries that have already been used several times before, so look to put your own unique spin on things before doing the bulk of the work.
As far as submitting the puzzles, every outlet has pretty tough competition. Will Shortz recently wrote that he accepts only 10% of all submissions
that come across his desk. I don't know what the numbers are for other venues, but I doubt the chances are significantly higher. I wouldn't let that dissuade you from giving it a shot -- crossword construction is pretty fun and editors do love to encourage new constructors to submit their work -- just know that the competition is pretty fierce. That's why it's all the more critical to make sure your puzzles really shine with great lively answers and clues, and an absolute minimum of dreck fill.
Hope that all helps.