Covering SEO in this short space is a little tough, but let me at least point you in the correct directions:
1) You will definitely want to use a CMS based platform like: Wordpress, Joomla or Drupal. Many of these are easily installed through your hosting provider's control panel.
2) For many of these, you can get many free templates which are quite modifiable...they all allow easy access to META tags and editing of page content - which is where 90% of SEO happens.
3) The CSS doesn't make much difference for SEO, your content does, so here is what I'm going to recommend:
a) Go to GOOGLE KEYWORD TOOLS (login with your gmail account...if you don't have one, get one).
b) Type in KEYWORDS that YOU THINK OTHERS ARE TYPING IN TO TRY AND FIND A SITE LIKE YOURS.
c) The tool will return Keywords that people are actually using...pick the best 1-3 keywords and try to optimize your home page for that.
d) Optimization is about the RIGHT MIX of keywords on your page...making sure your site "complies" with the TOP 10 sites currently showing up in the Google search for a particular term.
Google looks at over 2000 pieces of information to rank you. There are really good pieces of software out there that can do analysis for you, but if you're on a budget, it may be a lot of trial an error. Keep in mind that another HUGE portion of SEO is backlinking (How many sites link to your site), the quality of those backlinks, age of your site, how long you've bought your domain for (longer the better). There is a lot to measure (make sure you install Google Analytics on your site as well...you can't see how you're doing if you aren't measuring).
Let me know if you run into roadblocks and I can give specific answers where applicable.