First step: the first improvisation

A typical question is: "where do I start from"?.
The typical answer is: "Let's decide some because lack of freedom = beginner's happines".

So the first exercice I would reccomend is to sit at your piano (organ, guitar, ...) and play a simple melody.

Before starting you have to choose:
1) a scale (like the C major scale)
2) a range (like from C to G on the same octave... I would suggest to start with a very narrow range, and to expand it only when you feel confident)
3) a rhytmic pattern (in 4/4 for example: three quarter notes and 2 8th notes)
4) a number of bars (it is important to decide HOW MANY BARS will you play) start with 4 bars, eventually moving to 8 or 16.
5) a tempo (e.g. 80bpm): it is important that YOU DECIDE the tempo, it must be a conscious choice.

Since these rules limit a lot the aesthetical result you should make short improvisations (so choose a short number of bars) to avoid monotony.

Then start:
count loud the bar numbers: ONE, TWO, THREE, ... I would suggest NOT TO COUNT the beats, instead feel them as if a drummer is playing in your head. Start with quarter note subdivisions (tic) than when you feel confortable move to 8th note subdivision (tac).
So you have to think
ONE tic tic tic TWO tic tic tic
and then
ONE tac tic tac tic tac tic tac TWO tac tic tac tic tac tic tac THREE...

At first don't care about which keys you press. Just make sure to hit the keys on the beat or subdivision (remember we are using a "3 quarter notes + 2 8th notes" pattern).
After some attempts start to pick the right notes, but this will come eventually...

And when you are confident use a rallentando at the end.

Ok, this is enough for the first attempt... Do not try to long on the same day, it is better to do this exercice every day for 5 minutes, instead of doing it for 1 hour once a week.

You'll see good progresses if you are constant.

Good luck!