高速道路 を 北 に 向かって 走っていた 私は、トイレ に 行きたく なった ので サービスエリア に 立ち寄りました。
高速道路 【こうそくどうろ】 highway, freeway
北 【きた】 (n) North
走る 【はしる】 (v5r) to run
私 【わたし】 (adj-no,n) I, myself, private affairs
トイレ (n) toilet, restroom, bathroom, lavatory
行く 【いく】 (v5k-s) to go
ので (suf) that being the case, because of ...
成る 【なる】 (v5r) (uk) to become
サービスエリア service area
立ち寄る 【たちよる】 (v5r) to stop by, to drop in for a short visit
行きたく is the adverb derived from 行きたい (want to go). Adverb us formed by dropping the final い and replacing it with a く
なった is the plain past form from the verb なる
As for I, who was facing north and running on the highway, due to developing a desire to go to the toilet, stopped at a service area.
Driving north on the highway, I stopped off at a service area because I had to go to the toilet.
There are a number of translations for this like
freeway, highway, interstate, speedway, and expressway.
I think that highway is the most universally understood, as freeway indicates that there are no tolls (Japanese highways are not free), and interstate suggests that you are traveling between states (no states in Japan, only prefectures).
Depending on what country you're from, you're going to call it something diffierent. Loo, toilet, bathroom, restroom, lavatory,john, etc. However, I think that in this case toilet (or restroom) is the best translation, as it is devoid of colloquialism and does not have the connotation of a connecting bath (bathroom). Lavatory sounds too formal, and "I have to go to the lavatory" just doesn't strike me as natural.
Just wanted to note that at least where I'm from in the northeastern US, we have both rest areas and service areas off our highways. Rest areas usually have a toilet and a few vending machines, while service areas generally have a gas station, a few fast food restaurants, and a convenience store. The "service area's" I've been to in Japan so far are more like what I think of as a service area than a rest area, with a gas station, places to eat, and of course places to buy おみやげ.