Oh dear, I can hardly believe that I'm turning 20 this year... (Comma! :3 In this case, because "oh dear" is an interjection, and it is separate from the rest of the sentence.)

(In Japan, 20 is the age where we are officially allowed to drink and smoke, and get married without our parents' consent.

We're finally regarded as a "grown-ups" when we've hit that age.) (To "hit an age" is an idiomatic expression. Note: "Hit the age of #" when specifying a number.)

This completely childish, shabby, lame-looking girl is turning twenty? (Commas because you're listing various descriptors. The girl who is childish and shabby and lame-looking.)

You know, my dad's passed away and all that.....

I have no clue how this year will turn out, but I do hope that I meet lots of lovely people who also think of me as a lovely person...(Does it even make sense? hehe) (In speech, you'll hear people use "that" instead of "who," but "who" is more correct, because you are talking about people.)

I've neglected writing here for the last five days...

The thing is, it feels so right to come back here and type whatever what I want to!

It feels as if it's a great way of liberating myself from the daily same old daily routine of communicating with people, which can be quite taxing at times. (I don't think you need the "daily," but if you want to add it...)

I'll just keep on doing whatever I love with my life.

From the past week's experience of my life, I've learned that every one of us is immortal, and so we have no time to waste... ("immortal" = can't die. "mortal" = can die.)

Whatever the hell it/that meaning might be ;)

To me, this year's already had one of the most hectic and hard bits occur.