Homesick to the point of tears.

Ok, yes, not EVERYONE is homesick, but the huge majority are to some degree.
If OP hasn't really been away from home before they are going to feel very weird about it and miss it. Everyone is different, maybe you adjust to things well and are a very secure and confident person.
Not everyone is, telling someone to 'man up' isn't going to magically get rid of overwhelming feelings of homesickness. Unless you've experienced those feelings you have no idea how horrible it can be.
There are coping mechanisms like getting involved and busy with uni and new people, but emotions aren't controlled by switches, you can't just think 'right tme to man up' and turn off all feelings of homesickness.

I disagree, it's all about your mental attitude, so taking a "man up" approach has to be one of the most effective ways of dealing with these feelings.

Certainly more effective than running home to your parents.

Ok, maybe I'm more disagreeing with the term 'man up'. If you mean if you go in thinking positively and focusing on uni life and the good things, rather than reflecting on what you miss, then you're more likely to be ok.

For me 'man up' implies to you expect someone to one day just leave all their emotions and get on with things and forget the feelings of missing home. Yes they can do the get on with things bit, but its not magically going to make them feel 100% better. Emotions aren't something you can easily control once they've kicked in.

I'm 18 and I've moved into uni this weekend. I'm incredibly homesick; I've never been away from home for more than 2 days and even then that was only 3 times a few years ago.

I know I need to 'man up' and 'just deal with it', but it's very hard for me to do. I miss the bed I've had all my life. I miss my family already and I'm about 2 hours away on the train from them. I know it's not that far away but realistically I can't see them that often.

I've been crying and I've been sick a few times. I can't stand not being at home; everything is so different and I've lost the security I used to have (the only place I've ever felt really safe is in my bedroom and now I don't have that any more).

What can I do?

Ok, first of all, don't dwell on how awful you are feeling because that just makes it ten times worse. My first day/night at uni was dreadful, everything reduced me to tears. Looking back on it a couple of days later, I can't work out whether it was homesickness or just the shock of changing environment so rapidly.
What I found that took my mind off things and cheered me up is going to the local shop and buying ingredients and just cooking for an hour or so. Its therapeutic and you have a yummy meal at the end of it.
If you don't want to go out, stay indoors. Its your uni experience, you do as you wish with it, don't feel pressured to do what everyone else is doing. My first night I went to the SU with some randomers and couldn't have felt worse about it, my anxiety kicked in so I left after an hour. These next couple of days I've stayed in for the evenings, yes I haven't met as many people as others have, but my flatmates are great and I'm already talking to a couple of people on my course.

To summarise, don't contemplate home life too much, and don't dwell on your current misery. You can do what you like at uni, just make sure if you stay in in the evenings make sure you collect as many contacts as possible from flatmates/coursemates etc. Don't panic!