I log:
- Nitrate
- Nitrite
- Ammonia
- Salinity
- Alkalinity
- Calcium
- Temperature
All on a weekly basis (of course some of those are for marine, but you can adjust with ones you've already got). Its good to get it going long term, because you can also note trends by using a graph function. For the basics, id use Microsoft Excel, its simple, and you might have encountered it before at school etc. There are also software programs specifically designed to log aquarium data, but I did some looking around, and the only good ones were pricey, and the free MS Excel does the job for me. SPSS is also another program you could use, but that does not come standard with MS packages, and you'd have to buy and install it yourself, and for the cost, and the time it would take you to learn all the functions, it wouldn't be worth it.