In English, we have a few ways of referring to time, relative to the current time. Mandarin is similar, and has a few patterns.
天 (Tiān) is day (although it can relate to the heavens, God, and much more)
Although 每个天 (Měi gè tiān) is literally "every (or each) day", doubling-up the character as 天天 (Tiāntiān) can also be used for every day
今天 (Jīntiān) is today
昨天 (Zuótiān) is yesterday
明天 (Míngtiān) is tomorrow (and roughly translates to "bright day")
前天 (Qiántiān) is the day before yesterday (and roughly translates to "in front day)
后天 (Hòutiān) is day after tomorrow (and roughly translates to "rear (or perhaps behind) day"
I'll write more soon on the best methods for learning Mandarin. If you want to get a taste for it in the meantime though, my current favorite site is iTalki, and my favorite teacher by far is Amy He. If you decide to try it, click here and it's cheaper for both you and me.