Using a tilde on a word doc.

It's in 'Symbols'... so 'Insert', go to 'Symbol', find the 'ñ', and there you go!

Another way is to use the Alt key plus a numerical combination on the square grid of numbers to the right on a standard keyboard (not the line of numbers across the top).

[FONT=Arial, sans-serif]ñ is Alt-164, for instance. You can do a wide variety of letters with diacritical marks, like é (Alt-130), etc., and other symbols, in this manner, using codes 129 and higher. In your word-processor, just make a document listing the coded symbols and save it. I find this very handy.
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I use autocorrect codes to put in my special symbols.

&dh = ð
&hd = Ð
&th = þ
&n~ = ñ

You will have to program them in and when you do punch them in, you have to press the space bar to execute and then the backspace to remove that unwanted space.

I use tildes all the time as per my mother tongue, and have a key with it for easy access. Even though I can do ñ, which we don't use in portuguese, I can't add it to a letter just because I feel like it.

Just out of curiosity, does the same happen to you? Would a numerical code overrun this?

The tilde only only as a few places in the character map. Only a few of them are allowed by the Alt code, the rest must be punched in by Unicode. If the keyboard and software agree, you can use the Compose key for easy access of them, but usually these are set by the language the software is set in.

à = 0195
ã = 0227
Ẫ = U-1EAA
ẫ = U-1EAB
Ẵ = U-1EB4
ẵ = U-1EB5
Ẽ = U-1EBC
ẽ = U-1EBD
Ễ = U-1EC4
ễ = U-1EC5
Ĩ = U-0128
ĩ = U-0129
Ñ = 0209
ñ = 0241
Õ = 0213
õ = 0245
Ỗ = U-1ED6
ỗ = U-1ED7
Ỡ = U-1EE0
ỡ = U-1EE1
Ũ = U-0168
ũ = U-0169
Ữ = U-1EEE
ữ = U-1EEF
Ỹ = U-1EF8
ỹ = U-1EF9