should I use 2 transistors for the switch of my step-up converter?
That isn't necessary (unless you want to use a MOSFET to replace diode D1 in order to get a few percent more efficiency).
how I can size the components of my step-up converter?
Well, you should decide what the maximum current out should be and, this should consider (usually) the stall current of your motors and the full load current. I see you have picked 500 Ω to represent the load and this represents a load power of 288 mW (i.e. 12 volts squared divided by 500 = 0.288) so, I'm guessing that you motors are quite small in power rating.
Your circuit will also be operating in what we call discontinuous conduction mode (DCM) as opposed to CCM (continuous conduction mode) with the values you have chosen. This basically mean that at the duty cycle used (0.66), the inductor current will naturally fall to zero amps towards the end of each switching cycle. That isn't a problem in itself because, the load is a motor but, for other (more precious) loads you may choose to operate in CCM. This would happen in your case if the inductor value rose to circa 190 μH.
I've developed this little web-based calculator that can show you the currents. It's not perfect (I should try to make it able to deal with smaller power levels) but it should help: -
Unfortunately, it barely shows the current waveforms but, it will give you something to play with (if you want to experiment with different values). It calculates the required duty cycle based on the input/output voltages, inductor and load resistance.
The web-page also shows you the formulas for deriving the info yourself.
To compute for example the inductance and the capacitance of my
converter, I have to set the frequency.
The capacitance plays only the role of ripple voltage reduction in a boost converter like this and, isn't relevant in deciding the operating frequency. However, inductance is important.
Realistically, you need to decide whether you want to run in CCM (better/lower ripple voltage) or DCM. Yours will be fine in DCM and, you could choose a lower value of inductor. On the other hand, if you are running in DCM it may be important to keep the peak current drawn from your unspecified battery to the lowest possible so, choosing a higher value inductor will reduce this. The fact that the battery is a button type does raise concern on its ability to deliver anything like 0.1 A.
In my picture above, the peak inductor current (that's the peak current drawn from the input supply) is 0.196 amps but, if you went for CCM and chose a 220 μH inductor, the peak current would be 0.133 amps. Again, using a button cell is a concern.