How do you know if you will use the -er or -ar endings with a verb? pls help

Well, you will know when to use -er when you refer as verbs with infinitives ending. What you will do is drop the -er from the infinitive and add the second conjugation present tense endings: -o, -es, -e, -emos, -en.

Explanation:

To use -ar you will use the first conjugation, for example, hablar, lavar, llamar.

If the verb is like cantar you will use these endings

canto

cantas

canta

cantamos

cantan

if it is like comer you will use

como

comes

come

comemos

comen

if it is like abrir you will use

abro

abres

abre

abrimos

abren

Explanation: