Each community and state seems to potentially handle these things differently.
Some require you to get a contractors license. Although a lot of times you don't need one if you are not doing improvements to the home. Simply cutting lawns many times doesn't require a license. You won't know for sure unless you visit your town hall or look it up online.
As Steve pointed out, your state may require you to collect sales tax for certain services. This is no big deal. Simply look up your state sales tax website and do a little research to see if it applies to what you are offering. Or talk to your accountant, they will know too.
Right now i mow at a low price but what do i do down the road to increase current clients montly cost with out making them mad or loosing there buisness?
This is a very good question. As you get new customers, can you raise your rates on them? With your older customers can you slowly raise their price over time?
Can you also look to offer upsells to them? Look around the yard and the house to see what needs to be worked on and offer to do that?