Get the same size strings I take it your on about electric I think there is like the normal strings and then there are the slinky ones that are slightly thinner i think if you put a diffrent sized set on you will need to reset the inotaton but I would recomend fine tuning this anyway if your changing strings.If you have a digital tuner it makes life so much easier if you dont have a look around the internet you can download some that uses your pc mic to tune but its not 100% but close enough.
If its a new guitar and your changing your strings for the first time I would just ask your guitar shop for regular electric strings, But if your not sure take your guitar into them or a broken string preff the high e the thin one and they will be able to tell what strings you have on...

I know im only a beginer my self but if you have any probs setting up and tuning give us a shout or post on here. all the guys are great here and always willing to help...