20mg would be a good starting dose.
I would not start with 20mg your very first time, more like 10mg, then if that turns out to not be enough you can get a better idea of an appropriate dose for next time from there (this is assuming it's regular release not extended release?). Some people require only a very small dose, some people require larger doses. Different people have very different reactions to it and I would start with a lower dose in case you are sensitive to it or you don't like it. You can read about my experience with Ritalin IR
here (I am one of those people who hated it, but note that my experience was worse than most). Some people love it and find it very effective, some people hate it. Make sure you are well-rested and well-hydrated and nourished.
Are you experienced with stimulants?
Are you using it therapeutically (for ADHD etc) or recreationally (in order to get high)?
What else would you like to know? Ritalin is a brand name for methylphenidate, which possesses some structural and pharmacological similarities to cocaine (although it's generally weaker). It is a stimulant, increasing dopamine, norepinephrine and serotonin.
Desirable effects can include: improved mood or euphoria, feeling more alert, energetic, focussed, productive, etc.
Side effects can include: nervousness/anxiety/restlessness, insomnia, back or abdominal pain, decreased appetite, raised heart rate, sweating, dry mouth, depression, etc.