The best way to start id fir do competitor analysis. Check from where their backlinks are coming and what keywords they are targeting. Do a deep analysis as it the important part.
Once its done try to get backlink from where they are getting if possible. Do onpage optimization for your site. Do offpage optimization like bookmarking , article submission, forums and blog posting and all. There are many thing but you have to choose what you have to use.
There are many factors which affect search engine optimization:-
Like domain age, how fast if your website, properly coded, seo urls , caconical urls , brroken links and all.
Always keep all the factors in mind and get it started.
What not to do:-
Sometime when people starts with search engine optimization they start posting on different forums and blog about their service and spam the board. This is a big disadvantage for your site and now a days you will see forums and blogs are no follow which makes spamming useless.
Always post comments which can be helpful for others and search engine will also give your site importance.
Never post n irrelevant sites like usually people do in seo that they start posting a web development service in a music forum. There is no use for that its also called as spamming only.
So make a brief plan and they start the optimization procedure..