There are actually more than one past but here we limit ourselves with the simple past (pretérito).  By all means learn the others but there's no point doing it before you know this tense by heart!

All regular verbs which end with -ar behave in the same way that CANTAR does:

CANTAR                    TO SING
Canté                         I sang
Cantaste                    You sang
Cantó                         S/He sang
Cantamos                  We sang
Cantasteis                 You sang
Cantaron                    They sang               

Note that we sing and we sang are both 'cantamos'.
Note that there is an accent on 'Cantó ' (S/He sang) to differewntiate it with 'Canto' (I sing).


All regular verbs which end with -er behave in the same way that COMER does:

COMER                    TO EAT
Comí                          I ate
Comiste                     You ate
Comió                        S/He ate
Comimos                    We ate
Comisteis                   You ate
Comieron                   They ate             

Beware the ending for they!

Unfortunately , verbs which end in -ir are mostly irregular.  There are rules but they apply rarely.  If you need an -ir verb, make sure it's a common one which you have learnt by heart.  If you have not, don't use it!