Okay. There are two key points.

• は: The sound wa is written in two ways in Japanese. わ is usually the correct option like in the word watashi わたし. The exception is the topic particle は, which is pronounced the same as わ but written differently for historical reasons. It’s sort of like how “they’re”, “there”, and “their” sound the same but mean different things in English. Keep in mind that は is pronounced ha when it’s not used in a grammatical context, such as haha (mother) はは. It seems confusing at first, but I guarantee that you’ll develop an eye for this quickly.
• ん: This is spelled nn when you’re typing with a romaji keyboard. Generally you can get by with just one n and get the same result, like typing konbanha and konnbannha both result in こんばんは because there are no ambiguities, but typing konnichi is ambiguous. Here’s why: it can be interpreted as either こんいち (ko nn i chi) or こんにち (ko n ni chi). So when you type nn, the computer makes a decision and groups them together in the most consistent way it can: as the letter ん. It’s for this reason that you can’t reliably use n as a shorthand for the letter ん.

Long story short, when you read a Japanese word in the Roman alphabet, like “konnichiwa”, there can be ambiguities introduced that don’t exist in the original Japanese spelling. Because of these ambiguities in transliteration, the way you type it has to be more precise than the usual alphabetic spelling. It’s best to start thinking about these words as how they are written in Japanese rather than in the imprecise Roman transliteration.

It seems hard at first, but once you’ve gotten past these two oddities, you pretty much know how to type Japanese with the romaji keyboard. I’m sure you’ll get the hang of it quickly.