“If” indicates a condition. If X then Y.
Different tenses and/or voices may be used inside part X (the condition) or part Y (the result), but some combinations are not meaningful together, or sometimes people think they are saying one thing, but they’re using the wrong tense or voice.
“If he walked into my life today, I would be happy.”
– This is a mis-match of tense and voice. “Walked” is past tense, which also doesn’t match “today”, which is present, so I assume that’s the mistake. It could be:
“If he were to walk into my life today, I would be happy.” – would be a correct match of subjunctive voice about the present. It might happen, and if it did, this is what I say I would feel about it.
“If he walk into my life, I will be happy” is also the wrong form – it looks like you meant the present form “walks”. This is declaring or asserting a fact, rather than using the subjunctive voice. It is simpler and more assertive said this way.
“If you did not create a user account in the firstboot screens, switch to a console by pressing Ctrl+Alt+F2” is ok. It says if you didn’t do something in the past, then do this in the present or future.
“If you do not create a user account in the firstboot screens, switch to a console by pressing Ctrl+Alt+F2” is all in the present, and so suggests that the condition and the resulting suggestion apply to the same time, rather than sequentially one before the other.
“If you tried to open home page, it would not get open.” Again this combined past tense condition with subjunctive voice, and in this context looks wrong. I don’t know what you’re trying to say, so I don’t know how to correct it.
“If you try to open home page, it will not open.” This is not using subjunctive so it is stating a conditional fact or assertion in the present/future tense, which is ok. It is telling someone that if they do something, the result will (not) be that the home page will open.