Hold down the Alt key and hit the Enter key.

If you want it to start in full screen mode, then you need to edit the dosbox.conf configuration file.

yea i know how to change it with notepad to rumthing. I jsut dont know what to change it to in the config t oget it to start on fullscreen.

Here's the beginning of my dosbox.conf file:

1[sdl]
2# fullscreen -- Start dosbox directly in fullscreen.
3# fulldouble -- Use double buffering in fullscreen.
4# fullfixed -- Don't resize the screen when in fullscreen.
5# fullresolution -- What resolution to use for fullscreen, use together with fullfixed.
6# output -- What to use for output: surface,overlay,opengl,openglnb,ddraw.
7# hwscale -- Extra scaling of window if the output device supports hardware scaling.
8# autolock -- Mouse will automatically lock, if you click on the screen.
9# sensitiviy -- Mouse sensitivity.
10# waitonerror -- Wait before closing the console if dosbox has an error.
11# priority -- Priority levels for dosbox: lower,normal,higher,highest.
12#             Second entry behind the comma is for when dosbox is not focused/minimized.
13# mapperfile -- File used to load/save the key/event mappings from.
14
15fullscreen=false
16fulldouble=true
17fullfixed=false
18fullresolution=1280x960
19output=ddraw
20hwscale=2.00
21autolock=true
22sensitivity=100
23waitonerror=true
24priority=higher,normal
25mapperfile=mapper.txt

You can see that the very first line (that doesn't start with a # symbol) controls whether or not DOSBox starts in full-screen mode. All you have to do is change it from "false" to "true".

Ive changed it but it still doesnt come up in fullscreen when i start it -.-;

Here are some things to try:
- Does Alt+Enter work at least?
- Change the output= line to one of the other listed rendering modes (the one you're trying to use might not support full screen mode?)
- You might also have a fullresolution= setting that isn't supported by your system (try 800x600 as a safe place to start)
- Try toggling the fullfixed= parameter

Be sure to post exactly any error messages you see in the DOSBox console window, and also exactly what you try and what effect it has.