I assume you meant “頑張りましょう” lol
Always use the 連用形 (い stem) of verbs before adding ます
頑張れましょう technically also exists but isnt really used. (Its Potential, Masu + Volitional)
Seing that you said you have some problems with conjugation, Ill try to explain it:
First of all, the western form of “conjugation” doesnt really exist in Japanese. Japanese uses helper words to add onto different stems of the original verb.
頑張る → Original verb form, indicated by the fact that it ends on a kana in the “う” row (In this case る)
As you know, Japanese uses 5 vowels, あ,い,う,え,お.
“る” falls under the “う” vowel row.
You can now take this “る” and replace the “う” with any of the aforementioned vowels. This is called a stem. (る → ら、り、る、れ、ろ)
When we want to add certain helper verbs, we need to first change the る into one of these stems. The reason for why we use a certain stem for a helper verb actually etymologically makes sense, but is a little too advanced to be learning instantly.
What is important to know is this:
For 五段 verbs:
(五段 verbs are most verbs that end with -う except ones that end with (-え)る and (-い)る)
ます uses the い stem and adds ます. (頑張る → 頑張り+ます)
Volitional uses the お stem and adds a う. (頑張る → 頑張ろ+う)
Potential uses the え stem and adds the verb る. (頑張る → 頑張れ+る)
Desireable uses the い stem and adds the adjective たい (頑張る → 頑張り+たい)
Receptive (Passive) uses あ stem and adds the verb れる (頑張る → 頑張ら+れる)
Causative uses the あ stem ands adds the verb せる (頑張る → 頑張ら+せる)
Negative uses the あ stem and adds the adjective ない (頑張る → 頑張ら+ない)
Hypothetical uses the え stem and adds ば (頑張る → 頑張れ+ば)
Imperative uses only the え stem. (頑張る → 頑張れ)
As you can see, these “Helper words” themselves are verbs/adjectives themselves, which means they can be conjugated further using the same principle.
The only (important) exceptions are:
する、ます、くる、いく、which all have unique conjugations that you need to learn.
For 一段 verbs:
(一段 are most (not all) verbs that end with (-え)る / (-い)る) like 食べる)
For these we simply cut the る completely and instantly add any of the helper words; These are much simpler since they dont have any stem and just add the helper word.
ます adds ます; (食べる-> 食べます)
Volitional adds よう. (食べる-> 食べよう)
Potential adds れる. (食べる-> 食べれる)
Desireable adds たい (食べる → 食べたい)
Receptive (Passive) adds られる (食べる → 食べられる)
Causative adds the させる (食べる → 食べさせる)
Negative adds ない (食べる → 食べない)
Hypothetical adds れば (食べる → 食べれば)
Imperative uses either ろ or よ (食べる → 食べろ / 食べよ)
Of course I didnt go into everything but that should suffice for the fundamentals.