Out of all the supps, besides protein, Creatine is the best bang for buck - it's cheap and has years of research to back it and works. Don't expect miracles from it though, you may notice a slight increase in strength and some people retain a bit of water from it's use.
Creatine Monohydrate will do the job. The others are more expensive and are SAID to be better when it comes to absorption rates etc.
Creatine is best taken with a carb source, I personally take it with Dextrose post workout in my shake. Thats probably the best time to take it with dextrose as the insulin spike will take the creatine straight to your muscles and replenish levels.
Typical dose is 3-5grams per day, some people "load" it for the first week by increase the dosage, it varies but 10grams usually split over twice throughout the day as it is said to load the muscles up with creatine and from the end of that week onwards the typical 3-5grams a dose is taken once a day to keep levels stable. Some people run it for around 8 weeks and discontinue use for a bit, say 4 weeks, then start an 8 week cycle. Loading/Cycling is optional, some people claim it's a must, others run it at 3-5grams daily and never come off it it's all down to personal choice. I personally load for the first week and run it for around 8 weeks.