#1: Finding the key
What U13 said Smiley
Finding the bassnote is ok, but it doesn't help you find the key if you don't know whether it's the tonic (root), 6, 5 or whichever.
The same applies for the melody note.
I usually use either the melody note or the bass to find the root note (I use the melody notes more though.) This helps you to establish the key you are in.
But it helps to be familiar with how most of the other tones sound like so that for a song that takes long to go to the root, you can establish what the root is , say, when you play & can identify a 4 note (sub dominant sound)..
#2: Finding chords to melody note.
99% of the time, you have to know the key you are in before you can add any chords to a melody note. This is because you have to know how the chord is acting--eg, is it a 1 chord or a 4 or 5 chord in that specific key you are playing in..So only add chords that you are certain with...
My second answer to this part is that you start off by playing slow in your practice-the focus should be on analyzing the chords you play for any song..look at the notes that make up the chord & listen to the sound they make & how that links up with the song you are playing.
Fret not because you will not have to do this forever--Once you've been playing for a while, & you've played various songs, you would have INCREASED YOUR CHORD BASE & THE INSTANCES WHEN EACH CHORD CAN BE USED.
#3: Playing behind another musician
What instrument will this other musician be on? I'm sure they'd most likelly be playing piano Undecided
So then you should be playing other sounds for nice effects--there are plenty of sounds you can play..
There are selections from the wind instruments (Whistle, Bottleblow, Panflute, flute, recorder etc), brass (mute trumpet, sax, french horns etc), synth pads etc: all of these don't require you to play chords--you merely harmonize based on what you are hearing in your head or how you thing the song can go in that given beat/ rhythm...I know it sounds terrible if I try to play chords with those voices..
So if you play with them, I advise you to listen to songs that uses similar sound effects..
I know that if I run out of ideas for instance, for fast songs, I just use the brass & as in beat as I can, I play a 5,6,1,6,1 [in Db: Ab,Bb,Db or Ab,Bb,Db,Bb,Db]
This is just 1 idea that works well for many fast songs (just like how the 1-4 bass runs work for just about any fast song).
The nice thing with brass also is that you don't have to play all the time--you can just play "in the spaces" or fill in every now & again..listen to more recordings with such stuff (Kirk's Brighter Day, lotsa Fred Hammond & many many more.)
Notice how all of the above requires you to play a 1 fingered note.
To play chords, you can use the string sounds as well (note how they fit both categories.) But remember to sustain the strings & make sure that you don't change chords too often as the piano would be doing..again, listening to recordings with strings will help you get a feel of what it should sound like/ can be done for dynamic effects.
The organs can also be used to play chords or single note runs..I love the organs coz they scream..& you can make it scream at the end of each verse by doing a run down from the 1 to the 5 [1,7,6,5] before a new verse/chorus starts. This one is the one an octave higher than where you'd be playing.
I hope these ideas make sense & will hep you to start exploring with new ideas. Smiley