Hard to play by yourself, realistically you need to make more than one character (you can swap them out during waves in the Build Phase).
Best to start with is probably a Tower Apprentice. After that you could try making a DPS Huntress or an Aura Monk.
You could also make a Tower Squire and pretty much solo most of the game yourself, but Apprentice Towers are much better in the long run.
If you want an easy head start you could pick up some Mana (in game money) with FG here. Just a warning, it goes pretty quick when you start upgrading things.