Hi. I know Japanese can be a little confusing some times but hang in there :) Words change their meanings depending on which Kanji you use. It's not essential at first, since even most Japanese people don't know when to use which characters in some cases but it's useful to know. My explanation started to get a bit too long so I chucked the unnecessary parts below. Basically if you use the kanji 頂きます it means you receive the gift and will eat/drink it but if you use 戴きます it just means you will receive it (saying you will receive it is sounds a bit strange in English when someone is gifting you but in Japan traditionally, you would be polite and say things like Oh, but I'm unworthy to be gifted such great gift. or something). いただきます is basically a politer form of もらいます and can written in two different Kanji, 頂きますor 戴きます. { Both Kanji have meanings related to higher ranks than the speaker, where 頂 describes the peak, or top of something and 戴 means to put something on top of your head.} いただきます is used when you're receiving something you didn't work for, or not enough for(most the time just to be polite by saying you're not worthy of the gift). There's even a way to say it using both Kanji which I feel it is the most polite form of the all. It's 頂戴（ちょうだい）します. ちょうだい is a word you say when you want something and basically means "Give me that(please)" Sorry I tend to explain too much but what I was saying its every thing depends on what Kanji you use. Hope this helped.

ありがとう is the "thank you" part although it is informal. いただきます is what you say before a meal, it is more along the lines of "let's eat". Thank you very much would be 1.どうも 2. ありがとう 3.ございました 1. Very 2. Thank you 3. this part is for formality/politeness. Hope this helps.