You need to take the TOEFL exam to prove that you have the required english level to study in an english speaking environment. Beside the TOEFL test some universities ask the student to take the TSE (Test of Spoken English) specially if you are offered a teaching assistant position or the TWE (Test of Written English). The majority of the universities however don't need these two tests.
You will not be able to get a visa if your TOEFL score is less than 550 (at least this is the case for Canada). However, the universities have even higher requirments. If you score more than 600 then you are more or less safe. If you are not sure of your ability then take the exam a little bit early so that you have the chance to take it a second time if your score is low. Note however that TOEFL scores are valid only for two years so don't take it too early.
GRE are on the other hand valid for 5
years. But GRE scores have another problem: all your test history is
indicated on the score sheet. So if you are not yet ready for GRE, don't
enter it and get a bad score since it will always appear in your score sheet.
GRE has a general test divided into 3 parts (verbal, quantitative and analytical) and several subject tests. The majority of the universities do not require the engineering subject test. Actually it is a very general test in basic engineering (fluids, thermodynamics, civil, electrical power, electronics, mechanics, ...). So to enter it you must study again everything you was tought in first and second year of the faculty which is not an easy task :-). Since it is so general the universities don't think it shows your ability to do postgraduate research in a specific field. On the other hand if you plan to go for computer `science' (not engineering) you will generally have to do a subject test in CS.
One thing to note is that GRE is strongly required by a wide majority of US univerisities and nearly totally ignored by Canadian universities. Anyway the more or less acceptable scores are: more than 750/800 for the quantitative part, this is quite easy for us engineers; more than 700/800 for the analytical, this is not hard to accomplish also and more than 400/800 for the verbal. This last part (verbal) is really the hardest to fulfill and usually the universities require higher scores but the egyptian english level (at least what I saw here at CUFE) does not exceed this score very much.
Don't worry if your scores are less than the above mentioned grades for TOEFL and GRE, universities can and do accept students having scores less than the published requirments. This, however, depends strongly on your other qualities (field of research, recommendation letters, availability of funding, a faculty professor who knows you personnaly, ...).
To register for any of these tests you should go to Amideast and ask for the dates and rates.