On my 110 normal circuits what size breaker should I use 20 amp or 15 amp the box is 100 amp service.

Depends on what wire gauge you are using or installing and what you plan to do with the wiring. 12 AWG is 20 Amp and 14 AWG is 15 Amp. Kitchen circuits are 20Amp and you need 2 by code.. You can have a mix of 15 and 20 depending on other appliances. Not a simple answer. 100Amp is barely sufficient for a small older home and is too low for AC and electric range, dryer etc. Consider an upgrade to at least 150 Amp.

Thank you

Leth where do you live, in the USA votage went to 120 decafdes ago. ma0641 is spot on. Here is a chart for future reference Ampacity Charts (http://www.cerrowire.com/default.aspx?id=46) If you are in the USA bedrooms now call for arc faults and wet areas GFCI but you may not be required to meet the new code if your changes are minor. If you are doing a major rewire or remodeling you'll need a permit and will likely be required to up grade