How to "market" my web software company?

I have been developing database-driven software since 1996. During the last year I decided to "sell" this software.

I've a lot of source code for a lot of ... customer needs. Starting from help desk software, to accounting etc.
Also I have a lot of good competence in e-commerce (I know a lot of open software), cms etc.

My main problem is: how to advertise my company? How much do I spend? And, which "way" do I "market" my company?
If Google Adwords, how much should I spend?

This is my very BIG problem ..

I think that the first question you should ask is not "how to advertise", but "to whom should I target my advertising". In order for marketing to work, it have to be really targeted and specific. You can't be everything to everybody. You have to propose some specific benefits that resonate with some problems that potential customer have.

Can you identify some people or companies that might be interested in what you propose? Who would benefit the most from your services or solutions? What are the solutions that you delivered again and again and who needed them? To whom your best solutions can provide the most value? the most ROI? Is there a customer segment where you can make a compelling case that one of you solution help the make more money?

Before advertising, you might want to talk to has much potential customer as possible in order to understand:
1. there needs and problems
2. their buying process (this is really important in B2B to understand how they buy: who will choose, who will use, who will pay, etc..)

I don't mean to avoid your question, BUT, I'm a strong believer in not trying to do everything yourself. If you are asking this question and you are already a good developer I strongly suggest you find someone else to help you with the sales and marketing. It doesn't make sense to to take the time to learn marketing when you have products ready to sell. Find someone who can help you with sales and marketing and start bringing in revenue NOW!!!

Best wishes...

Uh ah, the ultimate "how to"...if I could only knew...ok, if you are a developer and want to stay one, you better find someone that can work on marketing and sales for you.

It sounds as though you're still working to distill all this source code down into what will be your product(s). I recommend that you start with the those solutions for which you have multiple existing customers. You will need to tell the story behind your products and how/why they help solve a need.

Once you've done that, you need to figure out where your prospects currently look for answers to the problems you solve. I would start by searching Google and Twitter for both competition and prospects to get a sense of where your market lies. After you find out where your prospects are, I'm sure you will have more specific questions about how to get their attention, and it will be far easier to offer specific advice.

Hope this helps...