There are many options to market your business. Both Erik and AbbyOki gave good suggestions.
I'd be happy to give you more input but we should narrow your question down to a more specific area, so that the feedback you receive may be more applicable to you.
First and foremost, it's important to define where you want to focus your time, money and efforts - this is the basis of a marketing strategy. To build a strategy, ask yourself these type of questions:
- What is your monthly marketing budget? In other words, are you able to spend $100, 500, $1500 or more per month on marketing?
- What methods of marketing have currently been working for you? Word of mouth? Flyers? Facebook? Google Adwords? Etc.. Look at expanding on what is currently working before trying new things.
- Is your marketing budget large enough for using multiple types of marketing without spreading things too thin?
- What are your goals? Do you need business coming in right away or are you more interested in building a sustainable method of getting business so that you can take your company to the next level - more employees, more equipment, etc.
Those are just a few questions to help get things going on the right track.
One suggestion that can get you customers pretty quick is using Google Adwords, if it's done right. I suggest you educate yourself a bit on it before jumping into it, but it's a great way to get your feet wet with buying visitors to your site that are specifically looking for tree work.
As Erik said, having a well built website is important in the long run and to really grow you should put some effort into your SEO and Google Maps rankings.
An offline method that works real well is door flyers, go to specific neighborhoods and attach a flyer to each door in the area. The flyer doesn't have to be expensive to make but the information on it needs to be laid out well. In some areas you can find companies to hire and they will pass out your flyers for you - be careful of their reputation and make sure they actually pass them out. You'll also need to check with the city just in case you need any special permits for that type of advertising.
My strengths are in online marketing - if you have specific questions or what to give me more information, I can give you some pointers.