Hey CreativeMind
Damn, that's a big question. Arrangement is one of those things you never stop learning, and for me it begins with *really* listening to your favourite records. And by really listen, I mean listen with a critical ear. I used to *really* listen to all my favourite records, but it wasn't until I began producing my own recordings that I stopped and actually analysed those records. And what I finally heard totally changed the way I approached recording.
To break it down, here's a quick answer to each of your questions. Drums I usually base on my acoustic guitar strumming pattern. If it's a fingerpicked song, or played on piano, I'll try and pick out the natural accents for the kick & snare. Sometimes just playing around with loops will give you something unexpected and brilliant. So that's always worth trying.
Bass: rule of thumb (which I really enjoy breaking) is that the bass should follow the rhythm of the kick drum. If you lay down the bass first, there's your kick drum pattern. Bass notes generally revolve around the root of your chords, but it can get boring if you don't explore further, so play with arpeggios, and some ascending/ descending runs for maximum impact. Also, don't forget to vary the note lengths else it can get a bit boring down there.
Strings: these are hard. I certainly wouldn't play a chord in a high register as that will likely sound fake, thin and nasty. When writing string parts, it helps a lot to know a bit about the instruments. Your string section consists of Double Bass, Cello, Viola & Violin. Each has its own role, and its own range, and its own 'sweet spot'. If you study string scores, you'll notice that there's usually quite a lot of vertical space between the 4 instruments. So for example, your Double Bass might be playing C2, your cello C3 or 4, your viola G4 and your violins E5 - this will give you that big, epic string sound you're probably looking for. If you're planning on having the strings play a melody, a good trick is to split the violins into octaves, and then use the remaining instruments to fill out the harmony & rhythm. Also, avoid 'footballs' - this video will explain
Another thing worth bearing in mind. If you want your song to sound 'big' you need to plan your arrangement accordingly. If there's not enough in the bass or the treble, it won't likely sound big. If all your parts are played in the same register (C3-C4 for example) your song will sound thin and cramped. Try to avoid playing too much over the main vocal - instead, use the gaps in between phrases to bring out your arrangement details. If you search the web for song production tips or music production advice, or similar, you'll find more content than you can possibly digest. It's a long journey, and the best way to learn is by doing. So as long as you allow yourself to experiment, get it wrong sometimes, and keep at it, you'll get there in your own time.
Hope some of that helps - and good luck!