Amphetamine, Methamphetamine, and Ritalyn are all "Central Nervous System Stimulants. There are differences in the chemical composition, and some differences in suspected pharmacological action, although the PDR (Physicians Desk Reference) admits that the exact mechanisms of action are not fully understood for any of the three.
The main medical use of Amphetamine (Adderall, Methedrine), Methamphetamine (Desoxyn) in recent years has been for weight loss, although that use is deprecated recently since most people who lose weight while using them seem to gain it back quite quickly when the therapy is stopped. Both are sometimes also used to treat children with Attention Deficit/Hyperactivity, although newer drugs like Ritalin have largely displaced them for this use.
The PDR warns that both of these have a high potential for addiction, and also includes a warning that patients may seek them for "illicit use." Both Amphetamine and Methamphetamine carry warnings about effects on circulation (blood pressure and pulse rate changes) and respiratory systems.
A side effect that the PDR doesn't warn about, for amphetamine or methamphetamine, is that the patient risks being "mugged" for the pills, since they're well known and have a fairly high value among illicit users. Some pharmacies, and some insurance policies, restrict dispensing of these drugs to small quantities (no more than a 1 week supply, perhaps) since having larger quantities on hand is considered a risk for the patient.
Ritaline has significantly fewer warnings about side effects, and is apparently significantly less prone to addiction, although there is a warning against very long term use. Treatment of Attention Deficit/Hyperactivity in children is the principal use given. There is a warning about prescribing Ritalin to "certain personality types" who may be inclined to increase dosage beyond the amounts prescribed, and in very high doses effects similar to the other two drugs are possible, including addiction/dependence.
The risk of addiction to Ritalin is indicated as being quite small at normal therapeutic dosages. There is the possibility of "accomodation" where the effectiveness of the drug is reduced, so that higher doses become necessary to achieve the same result. You should, of course, discuss any changes in dosage with your doc.
The only warning unique to Ritalin is that prolonged use at high dosages appears to inhibit normal growth in some children, so when/if you reach adolescence you may be a bit scrawny, relative to what you might otherwise have been. This shouldn't be a problem if you don't intend to grow up (any more than you have already).