You have to construct towers, be carefull towers cost money : you can choice to add new ones or just upgrade the older one you already bought.
After what you can have new towers more powerful, you can sell your older towers too.
Click on "H" to see the life of your ennemies. Click on towers to have the upgrade menu. You should read the full help if you've never played to that sort of towers game...
It's a very cool game.
I maybe have not so much points but I know a good tips is not construct too manu towers but place them very well and upgrade them in the same area at first. After what you'll see how long your ennemies can walk and decide to place new towers

Have fun!

http://www.novelconcepts.co.uk/FlashElementTD/play.asp