"I travel by myself often."
Technically で can also be a verb, as the て-form or conjunctive form of だ/です, essentially meaning "am/are/is ... and".
Example sentence from Wiktionary:
Kore wa hon de, totemo omoshiroi desu.
This is a book, and it is very interesting.
It depends on the verb.
In this case the し is part of the full verb します, the polite form of the verb する "to do"
旅行 - travel (noun) します - to do (verb)
旅行します "to do travel/to travel"
As for other verbs, there are two main types of verbs; ichidan (those that end in iru/eru) and godan (those with any other ending, mu/bu/ku/aru/uru/oru/u/tsu)
For ichidan verbs, these simply drop the final る and add ます
食べる - taberu - to eat -> 食べます to eat (polite)
寝る - neru - to sleep -> 寝ます to sleep (polite)
着る - kiru - to wear -> 着ます to wear (polite)
いる - iru - to exist (animate) -> います - to exist (animate, polite)
Godan verbs change the final -u sound to an -i sound before adding ます
読む yomu - to read -> 読みます yomimasu - to read (polite)
遊ぶ asobu - to play -> 遊びます asobimasu - to play (polite)
行く iku - to go -> 行きます ikimasu - to go (polite)
話す hanasu - to speak -> 話します hanashimasu - to speak (polite) - the "shimasu" ending here is unrelated to the verb する
買うkau - to buy -> 買います kaimasu - to buy (polite)
売る uru - to sell -> 売ります urimasu - to sell (polite)
ある aru - to exist (inanimate) -> あります arimasu - to exist (inanimate, polite)
Then there are two exceptions which change not only their final u ending pronunciation but the entire word
来る - kuru - to come -> 来ます kimasu to come (polite)
する - suru - to do -> します shimasu - to do (polite)