In the case of sex with another male (or with anyone, for that matter), the risk of acquiring an STI (sexually transmitted infection, which some people call STDs) is greatly reduced when you use a condom; this applies to both oral and/or anal sex. I realize this is not the most exciting answer but it's best not to beat around the bush when it comes to STIs.
If you want to know specific STIs, you're most likely at risk for the most common among gay men are:
Know what's interesting about this list? Heterosexual sex (or sex between men and women, no matter what someone's orientation is) poses the same risk factors!
Personally, I like to think of STIs as nuclear warfare: they don't care who they hit, so long as they hit something. And just so we're all clear, using a condom during genital sex of any kind greatly reduces your exposure to these STIs. With infections spread by body fluids, like HIV and Chlamydia, condoms provide upwards of 90% protection. With skin-to-skin transmitted infections, like Herpes and HPV, protection is understood to be around 70%. No matter how you slice it, in either case, that's a lot of protection. Without using latex barriers, your protection is a whopping 0%.
(Or you can just wash your hands, give each other handjobs and call it a day.)
Now that we've all had a good scare with STIs, let's talk about how to enjoy sex with another male.
It's important to first establish that you find yourself sexually aroused by other men. Notice I didn't write all men. In time, you will find yourself looking for specific qualities in guys you want to kiss, touch, etc. If you are not attracted to a guy, nothing in this world will force your decision otherwise. You might find a guy attractive when he's standing right in front of you, but that could change when the clothes come flying off or that hot kiss you anticipated wasn't so hot after all. This has happened to me, this has happened to my friends, and it can happen to you. In that situation, your best bet is to gather your clothes, do your best to keep what little dignity you have remaining, and make tracks out of there. Tell only your closest friends or your private LiveJournal account.
But let's say that mutual attraction is strong (which rules out the previous paragraph). Have you had anything to drink or smoke prior to ripping off each other's clothes? Alcohol or drugs can cause men to lose, or fail to achieve, an erection. I know there's a term for impotence due to drinking but I can't think of it at the moment. I'm sure it'll come to me later.
So if you're both naked, sober, and horny for one another, it could be that you're too nervous to perform at your very best. Try taking some time to make each other laugh and relax. Remember that you're doing this to have fun. Your level of enjoyment will increase with your comfort level. If you need more time to get comfortable with a partner first -- whether that's minutes, hours, days or months -- it's okay: give yourself whatever time you need. Kissing and touching also go a long way. When it comes time to use a condom, make a game of it so you keep the good times going strong.