"I am going to Inverness today."
Translation:Tha mi a' dol gu Inbhir Nis an-diugh.
In Gaelic there isn't actually a present tense as such. Everything is either past tense or in some form of future tense. Tha is more immediate than Bidh. Bidh is also habitual. For example Bidh mi a' snamh Diciadain implies that you go swimming every Wednesday, not just the next Wednesday coming up.
Gaelic and English use the I am going structure in the same way. Of course you could use the future, but tj4234 explains why bidh is not ideal so you could use thèid 'will go' if you have learnt it yet. So in each language there are two ways, and it can be a bit unclear what exactly the difference is. So the simplest plan is to use tha mi a' dol for I am going and thèid mi for I will go.
Do'n (old spelling) or don (modern spelling) is a contraction of *do an 'to the'. But there is no 'the' in this sentence. You would only use it with a placename that had an article in it, such as An Gearasdan - 'the Garrison', 'Fort William', hence don Ghearasdan 'to Fort William'. Note it is not common to use gu with places that do start with an article like this.
Both do dh'Inbhir Nis and a dh'Inbhir Nis should be acceptable. Both do and a mean 'to', and the dh' is added before a vowel, for reasons that are not fully understood.