Is the Laughable Menace laughing at your efforts? Try one of these resources. You certainly don't need to consult all of them -- pick one or two that look good to you. Some are more detailed, some more concise. Each one of them is likely to help your game, if you're just starting out.
The Tower Wars Wiki is basically the game manual. It is available in-game via Main Menu --> Help, or here:LINK[wiki.towerwarsgame.com]
The main Guides page is available in-game via the shift-tab overlay, or here:LINK
Katsuni's new forum guide:LINK
Voyle's classic guide for towers:LINK
Voyle's classic guide for units:LINK
A multiplayer game played by MindALot -- here he plays against a human, not an AI, but the lessons are just as valid:LINK
Oblivion defeats Lucrum, bot 10 (twice):LINK
The main Videos page:LINK
Don't forget to use the Tutorial in-game! After that, Classic Tower Defense may be useful too -- it won't train you to send units, build mines or invest in tech, but if you want to gain some familiarity with just the towers first, you can play around there.