Firstly, you need to decide what you are going to wear on the big day. It’s wise to have a rough idea of the type of thing you want before you go out to the shops, and also take a look at some websites for inspiration (and prices). Traditionally, a tuxedo is worn in the US, and a morning suit in the UK. However, you don’t have to take this as something that you have to do – many wedding these days are becoming much more relaxed (which I think is great!). Wear what you feel comfortable in. If you are getting married abroad, in a hot climate, something more casual might be appropriate – you don’t want to be a sweaty mess whilst getting married!
Secondly, you will likely need to write (and present!) a Speech at the wedding reception. Some grooms think they don’t need to prepare for this but it’s not advisable. You should at least make a few notes, so that you remember to thank all the people that you need to thank (mainly the bride, bridesmaids and parents). The speech doesn’t need to go on too long and it’s not essential to make it funny (leave that to the best man). However, just stealing one liners from the internet can be dull for the audience who have heard it all before. There are many ways to make the speech more unique, by including props, videos, or making it rhyme.
Thirdly, you can be involved in planning the honeymoon. This is normally a joint task with the bride but you might want to take on the bulk of the work as she will have enough to do. Decide where to go, making sure you consider the weather, and start planning. Finally, choose your best man. It may be immediately obvious to you who this is going to be. If this isn’t the case, then make sure whoever you pick knows you well enough to give a speech about you and also make sure he is reliable. He’ll be arranging the stag night / bachelor party so you’ll also need to make sure he is going to be able to organise this. If you have a few choices of people to be best man, and are concerned about letting someone down, you could always have more than one best man (this lets them share their tasks), or you could ask the ones who you do let down to be your ushers (UK) or groomsmen (U.S.) They could even give a reading at the ceremony if they wanted to.